Improve CC_FOR_BUILD detection

This commits simply uses CC as CC_FOR_BUILD and the same for
corresponding flags if we're not cross-compiling. This has a number of
benefits in this common case:
 - It avoids strange cases where very old compilers are used (#768).
 - Flags are consistently set for CC and CC_FOR_BUILD.
 - ./configure is faster.
 - You get compiler x consistently if you set CC=x; we got this wrong
   in CI in the past.

./configure warns if a _FOR_BUILD variable is set but ignored because
we're not cross-compiling.

The change exposed that //-style comments are used in gen_context.c,
which is also fixed by this commit.

This commit also reorganizes code in configure.ac to have a cleaner
separation of sections.

configure.ac

@@ -22,7 +22,6 @@ PKG_PROG_PKG_CONFIG
 AC_PATH_TOOL(AR, ar)
 AC_PATH_TOOL(RANLIB, ranlib)
 AC_PATH_TOOL(STRIP, strip)
-AX_PROG_CC_FOR_BUILD
 
 AM_PROG_CC_C_O
 
@@ -394,56 +393,75 @@ fi
 
 # Handle static precomputation (after everything which modifies CFLAGS and friends)
 if test x"$use_ecmult_static_precomputation" != x"no"; then
-  # Temporarily switch to an environment for the native compiler
+  if test x"$cross_compiling" = x"no"; then
-  save_cross_compiling=$cross_compiling
-  cross_compiling=no
-  SAVE_CC="$CC"
-  CC="$CC_FOR_BUILD"
-  SAVE_CFLAGS="$CFLAGS"
-  CFLAGS="$CFLAGS_FOR_BUILD"
-  SAVE_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="$CPPFLAGS_FOR_BUILD"
-  SAVE_LDFLAGS="$LDFLAGS"
-  LDFLAGS="$LDFLAGS_FOR_BUILD"
-
-  warn_CFLAGS_FOR_BUILD="-Wall -Wextra -Wno-unused-function"
-  saved_CFLAGS="$CFLAGS"
-  CFLAGS="$warn_CFLAGS_FOR_BUILD $CFLAGS"
-  AC_MSG_CHECKING([if native ${CC_FOR_BUILD} supports ${warn_CFLAGS_FOR_BUILD}])
-  AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
-      [ AC_MSG_RESULT([yes]) ],
-      [ AC_MSG_RESULT([no])
-        CFLAGS="$saved_CFLAGS"
-      ])
-
-  AC_MSG_CHECKING([for working native compiler: ${CC_FOR_BUILD}])
-  AC_RUN_IFELSE(
-    [AC_LANG_PROGRAM([], [])],
-    [working_native_cc=yes],
-    [working_native_cc=no],[:])
-
-  CFLAGS_FOR_BUILD="$CFLAGS"
-
-  # Restore the environment
-  cross_compiling=$save_cross_compiling
-  CC="$SAVE_CC"
-  CFLAGS="$SAVE_CFLAGS"
-  CPPFLAGS="$SAVE_CPPFLAGS"
-  LDFLAGS="$SAVE_LDFLAGS"
-
-  if test x"$working_native_cc" = x"no"; then
-    AC_MSG_RESULT([no])
-    set_precomp=no
-    m4_define([please_set_for_build], [Please set CC_FOR_BUILD, CFLAGS_FOR_BUILD, CPPFLAGS_FOR_BUILD, and/or LDFLAGS_FOR_BUILD.])
-    if test x"$use_ecmult_static_precomputation" = x"yes";  then
-      AC_MSG_ERROR([native compiler ${CC_FOR_BUILD} does not produce working binaries. please_set_for_build])
-    else
-      AC_MSG_WARN([Disabling statically generated ecmult table because the native compiler ${CC_FOR_BUILD} does not produce working binaries. please_set_for_build])
-    fi
-  else
-    AC_MSG_RESULT([yes])
     set_precomp=yes
+    if test x"${CC_FOR_BUILD+x}${CFLAGS_FOR_BUILD+x}${CPPFLAGS_FOR_BUILD+x}${LDFLAGS_FOR_BUILD+x}" != x; then
+      AC_MSG_WARN([CC_FOR_BUILD, CFLAGS_FOR_BUILD, CPPFLAGS_FOR_BUILD, and/or LDFLAGS_FOR_BUILD is set but ignored because we are not cross-compiling.])
+    fi
+    # If we're not cross-compiling, simply use the same compiler for building the static precompation code.
+    CC_FOR_BUILD="$CC"
+    CFLAGS_FOR_BUILD="$CFLAGS"
+    CPPFLAGS_FOR_BUILD="$CPPFLAGS"
+    LDFLAGS_FOR_BUILD="$LDFLAGS"
+  else
+    AX_PROG_CC_FOR_BUILD
+
+    # Temporarily switch to an environment for the native compiler
+    save_cross_compiling=$cross_compiling
+    cross_compiling=no
+    SAVE_CC="$CC"
+    CC="$CC_FOR_BUILD"
+    SAVE_CFLAGS="$CFLAGS"
+    CFLAGS="$CFLAGS_FOR_BUILD"
+    SAVE_CPPFLAGS="$CPPFLAGS"
+    CPPFLAGS="$CPPFLAGS_FOR_BUILD"
+    SAVE_LDFLAGS="$LDFLAGS"
+    LDFLAGS="$LDFLAGS_FOR_BUILD"
+
+    warn_CFLAGS_FOR_BUILD="-Wall -Wextra -Wno-unused-function"
+    saved_CFLAGS="$CFLAGS"
+    CFLAGS="$warn_CFLAGS_FOR_BUILD $CFLAGS"
+    AC_MSG_CHECKING([if native ${CC_FOR_BUILD} supports ${warn_CFLAGS_FOR_BUILD}])
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
+        [ AC_MSG_RESULT([yes]) ],
+        [ AC_MSG_RESULT([no])
+          CFLAGS="$saved_CFLAGS"
+        ])
+
+    AC_MSG_CHECKING([for working native compiler: ${CC_FOR_BUILD}])
+    AC_RUN_IFELSE(
+      [AC_LANG_PROGRAM([], [])],
+      [working_native_cc=yes],
+      [working_native_cc=no],[:])
+
+    CFLAGS_FOR_BUILD="$CFLAGS"
+
+    # Restore the environment
+    cross_compiling=$save_cross_compiling
+    CC="$SAVE_CC"
+    CFLAGS="$SAVE_CFLAGS"
+    CPPFLAGS="$SAVE_CPPFLAGS"
+    LDFLAGS="$SAVE_LDFLAGS"
+
+    if test x"$working_native_cc" = x"no"; then
+      AC_MSG_RESULT([no])
+      set_precomp=no
+      m4_define([please_set_for_build], [Please set CC_FOR_BUILD, CFLAGS_FOR_BUILD, CPPFLAGS_FOR_BUILD, and/or LDFLAGS_FOR_BUILD.])
+      if test x"$use_ecmult_static_precomputation" = x"yes";  then
+        AC_MSG_ERROR([native compiler ${CC_FOR_BUILD} does not produce working binaries. please_set_for_build])
+      else
+        AC_MSG_WARN([Disabling statically generated ecmult table because the native compiler ${CC_FOR_BUILD} does not produce working binaries. please_set_for_build])
+      fi
+    else
+      AC_MSG_RESULT([yes])
+      set_precomp=yes
+    fi
   fi
+
+  AC_SUBST(CC_FOR_BUILD)
+  AC_SUBST(CFLAGS_FOR_BUILD)
+  AC_SUBST(CPPFLAGS_FOR_BUILD)
+  AC_SUBST(LDFLAGS_FOR_BUILD)
 else
   set_precomp=no
 fi
@@ -559,3 +577,9 @@ echo "  CFLAGS                  = $CFLAGS"
 echo "  CPPFLAGS                = $CPPFLAGS"
 echo "  LDFLAGS                 = $LDFLAGS"
 echo
+if test x"$set_precomp" = x"yes"; then
+echo "  CC_FOR_BUILD            = $CC_FOR_BUILD"
+echo "  CFLAGS_FOR_BUILD        = $CFLAGS_FOR_BUILD"
+echo "  CPPFLAGS_FOR_BUILD      = $CPPFLAGS_FOR_BUILD"
+echo "  LDFLAGS_FOR_BUILD       = $LDFLAGS_FOR_BUILD"
+fi

src/gen_context.c

@@ -4,8 +4,8 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
-// Autotools creates libsecp256k1-config.h, of which ECMULT_GEN_PREC_BITS is needed.
+/* Autotools creates libsecp256k1-config.h, of which ECMULT_GEN_PREC_BITS is needed.
-// ifndef guard so downstream users can define their own if they do not use autotools.
+   ifndef guard so downstream users can define their own if they do not use autotools. */
 #if !defined(ECMULT_GEN_PREC_BITS)
 #include "libsecp256k1-config.h"
 #endif