Merge branch 'rs/use-enhanced-bre-on-macos' into maint-2.39

Newer regex library macOS stopped enabling GNU-like enhanced BRE,
where '\(A\|B\)' works as alternation, unless explicitly asked with
the REG_ENHANCED flag.  "git grep" now can be compiled to do so, to
retain the old behaviour.

* rs/use-enhanced-bre-on-macos:
  use enhanced basic regular expressions on macOS

git-compat-util.h

@@ -1346,6 +1346,11 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
 	return regexec(preg, buf, nmatch, pmatch, eflags | REG_STARTEND);
 }
 
+#ifdef USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+int git_regcomp(regex_t *preg, const char *pattern, int cflags);
+#define regcomp git_regcomp
+#endif
+
 #ifndef DIR_HAS_BSD_GROUP_SEMANTICS
 # define FORCE_DIR_SET_GID S_ISGID
 #else