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
 
@@ -394,6 +393,19 @@ fi
 
 # Handle static precomputation (after everything which modifies CFLAGS and friends)
 if test x"$use_ecmult_static_precomputation" != x"no"; then
+  if test x"$cross_compiling" = x"no"; then
+    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=$cross_compiling
     cross_compiling=no
@@ -444,6 +456,12 @@ if test x"$use_ecmult_static_precomputation" != x"no"; then
       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