MacOS: workaround compiler errors and case-insensitivity

MacOS: workaround compiler errors and case-insensitivity

MacOS GCC version implicitly searches the local directory for .h includes
despite the absence of -I. in the provided options.
Furthermore it searches with case-insensitive filenames due to the
underlying case-insensitive filesystem.

The combined result is that libacl .cc files include their local copy of
acl/Url.h instead of the base directories src/URL.h which was needed.

The long term fix will be to shuffle URL.h and its related code into
a convenience library. For now we can avoid issues by prefixing the full
src/ path to the includes.