let clang use the constant-return error() macro

Commit e208f9c converted error() into a macro to make its constant return value more apparent to calling code. Commit 5ded807 prevents us using this macro with clang, since clang's -Wunused-value is smart enough to realize that the constant "-1" is useless in some contexts. However, since the last commit puts the constant behind an inline function call, this is enough to prevent the -Wunused-value warning on both modern gcc and clang. So we can now re-enable the macro when compiling with clang. Tested with clang 3.3, 3.4, and 3.5. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>