Allow building with Homebrew apache24

This pull request fixed #401: Compile php54, php53 against homebrew apache24.

The issue is that when `inreplace` Makefile to use Homebrew prefix for the Apache libexec folder, the text to be replaced is hardcoded with OSX shipped apache, and system apxs.

I've just replaced the hardcoded string with a more flexible regexp to retain current apache config path and homebrew apxs but replace `LIBEXECDIR` with correct one.

I also removed `apache_libexec` because the module is now correctly complied to libexec dir of the installing php. This method creates another unnecessary caveat of moving the compiled module after installation.

@LukeCarrier: could you please confirm that if we can remove `apache_libexec` in favor of outputting the file directly to installing php's libexec folder?