gettext: add is_utf8_locale()

This function returns true if git is running under an UTF-8
locale. pcre in the next patch will need this.

is_encoding_utf8() is used instead of strcmp() to catch both "utf-8"
and "utf8" suffixes.

When built with no gettext support, we peek in several env variables
to detect UTF-8. pcre library might support utf-8 even if libc is
built without locale support.. The peeking code is a copy from
compat/regex/regcomp.c

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gettext.h

@@ -90,5 +90,6 @@ const char *Q_(const char *msgid, const char *plu, unsigned long n)
 #endif
 
 const char *get_preferred_languages(void);
+extern int is_utf8_locale(void);
 
 #endif