autoconf: #include <libintl.h> when checking for gettext()

Some libc implementations like uclibc or musl provides
gettext stubs via libintl library but this case is not checked
by AC_CHECK_LIB(c, gettext ...) because gcc has gettext as builtin
which passess the check.

So check it with included libintl.h where gettext may unfold into
libintl_gettext which will cause check to fail if libintl_gettext are
needed to be linked with -lintl.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

configure.ac

@@ -763,9 +763,19 @@ AC_CHECK_LIB([c], [basename],
 GIT_CONF_SUBST([NEEDS_LIBGEN])
 test -n "$NEEDS_LIBGEN" && LIBS="$LIBS -lgen"
 
-AC_CHECK_LIB([c], [gettext],
-[LIBC_CONTAINS_LIBINTL=YesPlease],
-[LIBC_CONTAINS_LIBINTL=])
+AC_DEFUN([LIBINTL_SRC], [
+AC_LANG_PROGRAM([[
+#include <libintl.h>
+]],[[
+char *msg = gettext("test");
+]])])
+
+AC_MSG_CHECKING([if libc contains libintl])
+AC_LINK_IFELSE([LIBINTL_SRC],
+	[AC_MSG_RESULT([yes])
+	LIBC_CONTAINS_LIBINTL=YesPlease],
+	[AC_MSG_RESULT([no])
+	LIBC_CONTAINS_LIBINTL=])
 GIT_CONF_SUBST([LIBC_CONTAINS_LIBINTL])
 
 #