When checking for gcc, use variable ac_cv_prog_gcc instead of GCC.

This gets AP_DEBUG set properly for --enable-maintainer-mode builds.

(As libtool configuration is removed from other places -- e.g., pcre,
expat -- the same change will be needed of those configuration
scripts use GCC.)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89084 13f79535-47bb-0310-9956-ffa450edef68