macos: do let the build find the gettext headers/libraries/msgfmt

Apparently a recent Homebrew update now installs `gettext` into a
subdirectory under /usr/local/, requiring the CFLAGS/LDFLAGS to list
explicit directories _even_ when asking to force-link the `gettext`
package.

Likewise, the `msgfmt` tool is no longer in the `PATH`.

Let's work around this issue.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -133,8 +133,11 @@ ifeq ($(uname_S),Darwin)
 	HAVE_BSD_SYSCTL = YesPlease
 	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 	HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
-	BASIC_CFLAGS += -I/usr/local/include
-	BASIC_LDFLAGS += -L/usr/local/lib
+	BASIC_CFLAGS += -I/usr/local/include -I/usr/local/opt/gettext/include
+	BASIC_LDFLAGS += -L/usr/local/lib -L/usr/local/opt/gettext/lib
+	ifeq ($(shell test -x /usr/local/opt/gettext/bin/msgfmt && echo y),y)
+		MSGFMT = /usr/local/opt/gettext/bin/msgfmt
+	endif
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease