V0.8: Fix autoconf warning.

This just copies the same change that was done for development over to V0.8.

aclocal.m4

@@ -25,7 +25,7 @@ AC_SUBST(install_suffix)
 #   - the object file produced by AC_COMPILE_IFELSE is called "conftest.$ac_objext"
 #   - the nm(1) utility is available, and its name is "nm".
 AC_DEFUN([_AX_C_UNDERSCORES_MATCH_IF],
-[AC_COMPILE_IFELSE([void underscore(void){}],
+[AC_COMPILE_IFELSE([AC_LANG_SOURCE([void underscore(void){}])],
 [AS_IF([nm conftest.$ac_objext|grep $1 >/dev/null 2>/dev/null],[$2],[$3])],
 [AC_MSG_ERROR([underscore test crashed])]
 )])