Add some CFLAGS in maintainer mode if supported by gcc:

-Wformat
-Wformat-security
-Werror=format-security
-Werror=declaration-after-statement

Add new macro APACHE_ADD_GCC_CFLAGS to do the compatibility check.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1180331 13f79535-47bb-0310-9956-ffa450edef68