Build: Silence two Autoconf warnings.

There were two uses of AC_COMPILE_IFELSE that didn't use
AC_LANG_SOURCE and Autoconf warned about these. The omission
had been intentional but it turned out that this didn't do
what I thought it would.

Autoconf 2.71 manual gives an impression that AC_LANG_SOURCE
inserts all #defines that have been made with AC_DEFINE so
far (confdefs.h). The idea was that omitting AC_LANG_SOURCE
would mean that only the exact code included in the
AC_COMPILE_IFELSE call would be compiled.

With C programs this is not true: the #defines get added without
AC_LANG_SOURCE too. There seems to be no neat way to avoid this.
Thus, with the C language at least, adding AC_LANG_SOURCE makes
no other difference than silencing a warning from Autoconf. The
generated "configure" remains identical. (Docs of AC_LANG_CONFTEST
say that the #defines have been inserted since Autoconf 2.63b and
that AC_COMPILE_IFELSE uses AC_LANG_CONFTEST. So the behavior is
documented if one also reads the docs of macros that one isn't
calling directly.)

Any extra code, including #defines, can cause problems for
these two tests because these tests must use -Werror.
CC=clang CFLAGS=-Weverything is the most extreme example.
It enables -Wreserved-macro-identifier which warns about
#define __EXTENSIONS__ 1 because it begins with two underscores.
It's possible to write a test file that passes -Weverything but
it becomes impossible when Autoconf inserts confdefs.h.

So this commit adds AC_LANG_SOURCE to silence Autoconf warnings.
A different solution is needed for -Werror tests.

1 files changed, 4 insertions, 5 deletions

diff --git a/configure.ac b/configure.ac
index 78d6e8ac..83d28b85 100644
--- a/configure.ac
+++ b/configure.ac
@@ -834,15 +834,14 @@ AC_C_BIGENDIAN
 # Use -Werror because some compilers accept unknown attributes and just
 # give a warning. If it works this should give no warnings, even
 # clang -Weverything should be fine.
-# dnl This doesn't need AC_LANG_SOURCE, minimal code is enough.
 AC_MSG_CHECKING([if __attribute__((__constructor__)) can be used])
 have_func_attribute_constructor=no
 OLD_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -Werror"
-AC_COMPILE_IFELSE([
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 	__attribute__((__constructor__))
 	static void my_constructor_func(void) { return; }
-], [
+]])], [
 	AC_DEFINE([HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR], [1],
 		[Define to 1 if __attribute__((__constructor__))
 		is supported for functions.])
@@ -878,12 +877,12 @@ if test "x$enable_ifunc" = xyes ; then
 	OLD_CFLAGS="$CFLAGS"
 	CFLAGS="$CFLAGS -Werror"
 	AC_MSG_CHECKING([if __attribute__((__ifunc__())) can be used])
-	AC_COMPILE_IFELSE([
+	AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 		static void func(void) { return; }
 		static void (*resolve_func (void)) (void) { return func; }
 		void func_ifunc (void)
 				__attribute__((__ifunc__("resolve_func")));
-	], [
+	]])], [
 		AC_DEFINE([HAVE_FUNC_ATTRIBUTE_IFUNC], [1],
 			[Define to 1 if __attribute__((__ifunc__()))
 			is supported for functions.])