Makefile: refactor regex compat support

There was no tweakable knob to use the regex compat code; it
was embedded in the mingw build. Since other platforms may
want to use it, let's factor it out in the usual way for
build configuration knobs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -194,6 +194,8 @@ all::
 #
 # Define USE_NED_ALLOCATOR if you want to replace the platforms default
 # memory allocators with the nedmalloc allocator written by Niall Douglas.
+#
+# Define NO_REGEX if you have no or inferior regex support in your C library.
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -884,9 +886,10 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	USE_NED_ALLOCATOR = YesPlease
 	UNRELIABLE_FSTAT = UnfortunatelyYes
 	OBJECT_CREATION_USES_RENAMES = UnfortunatelyNeedsTo
-	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -DNOGDI -Icompat -Icompat/regex -Icompat/fnmatch
+	NO_REGEX = YesPlease
+	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -DNOGDI -Icompat -Icompat/fnmatch
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
-	COMPAT_OBJS += compat/mingw.o compat/fnmatch/fnmatch.o compat/regex/regex.o compat/winansi.o
+	COMPAT_OBJS += compat/mingw.o compat/fnmatch/fnmatch.o compat/winansi.o
 	EXTLIBS += -lws2_32
 	X = .exe
 ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
@@ -1200,6 +1203,10 @@ endif
 ifdef UNRELIABLE_FSTAT
 	BASIC_CFLAGS += -DUNRELIABLE_FSTAT
 endif
+ifdef NO_REGEX
+	COMPAT_CFLAGS += -Icompat/regex
+	COMPAT_OBJS += compat/regex/regex.o
+endif
 
 ifdef USE_NED_ALLOCATOR
        COMPAT_CFLAGS += -DUSE_NED_ALLOCATOR -DOVERRIDE_STRDUP -DNDEBUG -DREPLACE_SYSTEM_ALLOCATOR -Icompat/nedmalloc