Build: Silence Autoconf warning.

There was a use of AC_COMPILE_IFELSE that didn't use
AC_LANG_SOURCE and Autoconf warned about this. 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
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, 2 insertions, 3 deletions

diff --git a/configure.ac b/configure.ac
index 45c74776..9eb17ac2 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.])