fix clang -Wunused-value warnings for error functions

Commit a469a101 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: NMax Horn <max@quendi.de>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -1136,7 +1136,7 @@ extern int check_repository_format_version(const char *var, const char *value, v
 extern int git_env_bool(const char *, int);
 extern int git_config_system(void);
 extern int config_error_nonbool(const char *);
-#ifdef __GNUC__
+#if defined(__GNUC__) && ! defined(__clang__)
 #define config_error_nonbool(s) (config_error_nonbool(s), -1)
 #endif
 extern const char *get_log_output_encoding(void);

git-compat-util.h

@@ -295,7 +295,7 @@ extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2)))
  * behavior. But since we're only trying to help gcc, anyway, it's OK; other
  * compilers will fall back to using the function as usual.
  */
-#ifdef __GNUC__
+#if defined(__GNUC__) && ! defined(__clang__)
 #define error(fmt, ...) (error((fmt), ##__VA_ARGS__), -1)
 #endif
 

parse-options.h

@@ -177,7 +177,7 @@ extern NORETURN void usage_msg_opt(const char *msg,
 
 extern int optbug(const struct option *opt, const char *reason);
 extern int opterror(const struct option *opt, const char *reason, int flags);
-#ifdef __GNUC__
+#if defined(__GNUC__) && ! defined(clang)
 #define opterror(o,r,f) (opterror((o),(r),(f)), -1)
 #endif