fix clang -Wunused-value warnings for error functions

fix clang -Wunused-value warnings for error functions

Commit a469a10 wraps some error calls in macros to give the
compiler a chance to do more static analysis on their
constant -1 return value.  We limit the use of these macros
to __GNUC__, since gcc is the primary beneficiary of the new
information, and because we use GNU features for handling
variadic macros.

However, clang also defines __GNUC__, but generates warnings
with -Wunused-value when these macros are used in a void
context, because the constant "-1" ends up being useless.
Gcc does not complain about this case (though it is unclear
if it is because it is smart enough to see what we are
doing, or too dumb to realize that the -1 is unused).  We
can squelch the warning by just disabling these macros when
clang is in use.

Signed-off-by: Max Horn <max@quendi.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>