a469a101 (silence some -Wuninitialized false positives;
2012-12-15) triggered "unused value" warnings when the return value of
opterror() and several other error-related functions was not used.
5ded807f (fix clang -Wunused-value warnings for error functions;
2013-01-16) applied a fix by adding #if !defined(__clang__) in cache.h
and git-compat-util.h, but misspelled it as #if !defined(clang) in
parse-options.h. Fix this.

This mistake went unnoticed because existing callers of opterror()
utilize its return value.  11588263 (parse-options: add
OPT_CMDMODE(); 2013-07-30), however, adds a new invocation of opterror()
which ignores the return value, thus triggering the "unused value"
warning.
Signed-off-by: NEric Sunshine <sunshine@sunshineco.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>