macOS: make sure that gettext is found

Due to reasons (some XCode versions seem to include gettext, some
don't?), Homebrew does not expose the libraries and headers in
/usr/local/ by default anymore.

Let's help find them again.

Note: for some reason, this is a change of behavior caused by the
upgrade to Mojave, identified in our Azure Pipeline; it seems that
Homebrew used to add the /usr/local/ directories to the include and link
search path before, but now it no longer does.

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

config.mak.uname

@@ -114,6 +114,8 @@ ifeq ($(uname_S),Darwin)
 	HAVE_BSD_SYSCTL = YesPlease
 	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 	HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
+	BASIC_CFLAGS += -I/usr/local/include
+	BASIC_LDFLAGS += -L/usr/local/lib
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease