Merge #584: configure: Use CFLAGS_FOR_BUILD when checking native compiler

a34bcaa Actually pass CFLAGS_FOR_BUILD and LDFLAGS_FOR_BUILD to linker (Tim Ruffing)
2d5f4ce configure: Use CFLAGS_FOR_BUILD when checking native compiler (Tim Ruffing)

Pull request description:

  This fixes a bug where configure would fail or disable static
  ecmult tables because it wrongly checks the native compiler using
  the target CFLAGS (instead of the native CFLAGS_FOR_BUILD).

  Moreover, this commit adds tests to figure out whether the native
  compiler supports the warning flags passed during the build, and it
  contains a few minor improvements to the code that checks the native
  compiler.

Tree-SHA512: 31a92a5516cf2f9801c918edfba0059aa4f8549b0c1de94fc166b5e92ad1868a480c48cdc5ff62679ba20e26f4a0e2948c71fd2b3e80766673d2bf7512da3875

Makefile.am

@@ -151,7 +151,6 @@ endif
 
 if USE_ECMULT_STATIC_PRECOMPUTATION
 CPPFLAGS_FOR_BUILD +=-I$(top_srcdir)
-CFLAGS_FOR_BUILD += -Wall -Wextra -Wno-unused-function
 
 gen_context_OBJECTS = gen_context.o
 gen_context_BIN = gen_context$(BUILD_EXEEXT)
@@ -159,7 +158,7 @@ gen_%.o: src/gen_%.c
 	$(CC_FOR_BUILD) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
 
 $(gen_context_BIN): $(gen_context_OBJECTS)
-	$(CC_FOR_BUILD) $^ -o $@
+	$(CC_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(LDFLAGS_FOR_BUILD) $^ -o $@
 
 $(libsecp256k1_la_OBJECTS): src/ecmult_static_context.h
 $(tests_OBJECTS): src/ecmult_static_context.h

configure.ac

@@ -162,27 +162,54 @@ else
 fi
 
 if test x"$use_ecmult_static_precomputation" != x"no"; then
+  # Temporarily switch to an environment for the native compiler
   save_cross_compiling=$cross_compiling
   cross_compiling=no
-  TEMP_CC="$CC"
+  SAVE_CC="$CC"
   CC="$CC_FOR_BUILD"
-  AC_MSG_CHECKING([native compiler: ${CC_FOR_BUILD}])
+  SAVE_CFLAGS="$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="$CFLAGS $warn_CFLAGS_FOR_BUILD"
+  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: ${CC_FOR_BUILD}])
   AC_RUN_IFELSE(
-    [AC_LANG_PROGRAM([], [return 0])],
+    [AC_LANG_PROGRAM([], [])],
     [working_native_cc=yes],
     [working_native_cc=no],[dnl])
-  CC="$TEMP_CC"
+
+  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([${CC_FOR_BUILD} does not produce working binaries. Please set CC_FOR_BUILD])
+      AC_MSG_ERROR([native compiler ${CC_FOR_BUILD} does not produce working binaries. please_set_for_build])
     else
-      AC_MSG_RESULT([${CC_FOR_BUILD} does not produce working binaries. Please set CC_FOR_BUILD])
+      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([ok])
+    AC_MSG_RESULT([yes])
     set_precomp=yes
   fi
 else