Make it right to check/implement va_copy

m4/ac_try_compile2.m4

@@ -0,0 +1,28 @@
+dnl Like AC_TRY_EVAL but also errors out if the compiler generates
+dnl _any_ output. Some compilers might issue warnings which we want
+dnl to catch.
+AC_DEFUN([AC_TRY_EVAL2],
+[{ (eval echo configure:__oline__: \"[$]$1\") 1>&AS_MESSAGE_LOG_FD; dnl
+(eval [$]$1) 2>&AS_MESSAGE_LOG_FD; _out=`eval [$]$1 2>&1` && test "x$_out" = x; }])
+
+dnl Like AC_TRY_COMPILE but calls AC_TRY_EVAL2 instead of AC_TRY_EVAL
+AC_DEFUN([AC_TRY_COMPILE2],
+[cat > conftest.$ac_ext <<EOF
+[#]line __oline__ "configure"
+#include "confdefs.h"
+[$1]
+int main(void) {
+[$2]
+; return 0; }
+EOF
+if AC_TRY_EVAL2(ac_compile); then
+  ifelse([$3], , :, [rm -rf conftest*
+  $3])
+else
+  echo "configure: failed program was:" >&AS_MESSAGE_LOG_FD
+  cat conftest.$ac_ext >&AS_MESSAGE_LOG_FD
+ifelse([$4], , , [  rm -rf conftest*
+  $4
+])dnl
+fi
+rm -f conftest*])