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 updated caveats to show the correct libexec path.