Use compatibility regex library for OSX/Darwin

Use compatibility regex library for OSX/Darwin

The standard libc regex library on OSX does not support alternation
in POSIX Basic Regular Expression mode.  This breaks the diff.funcname
functionality on OSX.

To fix this, we use the GNU regex library which is already present in
the compat/ diretory for the MinGW port.  However, simply adding compat/
to the COMPAT_CFLAGS variable causes a conflict between the system
fnmatch.h and the one present in compat/.  To remedy this, move the
regex and fnmatch functionality to their own subdirectories in compat/
so they can be included seperately.

Signed-off-by: Arjen Laarhoven <arjen@yaph.org>
Tested-by: Mike Ralphson <mike@abacus.co.uk> (AIX)
Tested-by: Johannes Sixt <johannes.sixt@telecom.at> (MinGW)
Signed-off-by: Junio C Hamano <gitster@pobox.com>