Does this do the trick?
(At least, the internet claims that's what apple calls it, and glibc
recognises it too)
Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/devel/gobject-introspection/Makefile,v
retrieving revision 1.61
diff -u -r1.61 Makefile
--- Makefile 5 Jun 2019 09:12:32 -0000 1.61
+++ Makefile 12 Jun 2019 12:40:18 -0000
@@ -34,7 +34,8 @@
SUBST_SED.timet+= -e 's,@TIMET_CTYPE@,glong,g'
.endif
-MAKE_ENV+= LD_LIBRARY_PATH=${WRKSRC}/output/girepository
+MAKE_ENV+= LD_LIBRARY_PATH=${WRKSRC}/output/girepository
+MAKE_ENV.Darwin+= DYLD_LIBRARY_PATH=${WRKSRC}/output/girepository
BUILDLINK_API_DEPENDS.glib2+= glib2>=2.58.0
.include "../../devel/glib2/buildlink3.mk"