compat/fopen.c: avoid clobbering the system defined fopen macro

compat/fopen.c: avoid clobbering the system defined fopen macro

Some systems define fopen as a macro based on compiler settings.
The previous technique for reverting to the system fopen function
by merely undefining fopen is inadequate in this case. Instead,
avoid defining fopen entirely when compiling this source file.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Tested-by: Mike Ralphson <mike@abacus.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>