use execute_process instead of exec_program in FindAPACHE.cmake

With this patch the [debian bug 707109] (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=707109) can be avoided; as apxs2 returns noise in debian wheezy, using exec_program the variable gets assigned the STDERR instead of the proper value. 

See also https://github.com/mapserver/mapserver/pull/4948