[PATCH] do not assume dead code existence in autoconf checks

when compiler optimize source, it removes dead code so a linkage error
in these cases are not visisble.

Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>

--- p11-kit-0.19.3/configure.ac	2013-07-24 11:13:03.000000000 +0300
+++ p11-kit-0.19.3.new/configure.ac	2013-07-27 23:05:53.470534878 +0300
@@ -100,9 +100,9 @@ if test "$os_unix" = "yes"; then
 	                AC_MSG_RESULT([yes])],
 	               [AC_MSG_RESULT([no])])
 	AC_CHECK_DECLS([__progname])
-	AC_LINK_IFELSE([AC_LANG_SOURCE([extern char *__progname; void main() { }])],
+	AC_LINK_IFELSE([AC_LANG_PROGRAM([[extern char *__progname]], [[__progname=NULL]])],
 		[AC_DEFINE(HAVE___PROGNAME, [1], [Whether __progname available])])
-	AC_LINK_IFELSE([AC_LANG_SOURCE([extern int __libc_enable_secure; void main() { }])],
+	AC_LINK_IFELSE([AC_LANG_PROGRAM([[extern int __libc_enable_secure;]], [[__libc_enable_secure = 0;]])],
 		[AC_DEFINE(HAVE___LIBC_ENABLE_SECURE, [1], [Whether __libc_enable_secure available])])
 fi