oauth: Fix Autoconf build on macOS

Oversight in b0635bfda. -lintl is necessary for gettext on Mac, which
libpq-oauth depends on via pgport/pgcommon. (I'd incorrectly removed
this change from an earlier version of the patch, where it was suggested
by Peter Eisentraut.)

Per buildfarm member indri.

src/interfaces/libpq-oauth/Makefile

@@ -47,7 +47,7 @@ $(stlib): override OBJS += $(OBJS_STATIC)
 $(stlib): $(OBJS_STATIC)
 
 SHLIB_LINK_INTERNAL = $(libpq_pgport_shlib)
-SHLIB_LINK = $(LIBCURL_LDFLAGS) $(LIBCURL_LDLIBS)
+SHLIB_LINK = $(LIBCURL_LDFLAGS) $(LIBCURL_LDLIBS) $(filter -lintl, $(LIBS))
 SHLIB_PREREQS = submake-libpq
 SHLIB_EXPORTS = exports.txt