Fix check for using computed goto flag for re2c

AC_LANG_SOURCE should be used instead of the AC_LANG_PROG so the
compilation check can succeed when the computed goto extension is
available to optimize conditional jumps.

configure.ac

@@ -178,7 +178,7 @@ if test "$PHP_RE2C_CGOTO" = "no"; then
   RE2C_FLAGS=""
 else
   AC_MSG_CHECKING([whether re2c -g works])
-  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [[
+  AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 int main(int argc, const char **argv)
 {
   argc = argc;