weechat: Use SUBST framework to adjust hardcoded pkg-config python checks

The previous ${LN} incantation missed possible python3-embed.pc file
(present in python38) ending up in a not-working python support.

Do not adjust file under ${BUILDLINK_DIR} and use the SUBST framework
to adjust all python3 pkg-config invocations.

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/chat/weechat/Makefile,v
retrieving revision 1.113
diff -u -p -r1.113 Makefile
--- Makefile	30 Mar 2020 11:07:18 -0000	1.113
+++ Makefile	21 Apr 2020 17:02:15 -0000
@@ -37,10 +37,12 @@ BUILDLINK_TRANSFORM+=	rm:-Werror-implici
 
 .include "options.mk"
 
-pre-configure:
 .if !empty(PKG_OPTIONS:Mpython)
-	${LN} -sf ${BUILDLINK_DIR}/lib/pkgconfig/python-${PYVERSSUFFIX}.pc \
-		${BUILDLINK_DIR}/lib/pkgconfig/python3.pc
+SUBST_CLASSES+=		python
+SUBST_STAGE.python=	pre-configure
+SUBST_MESSAGE.python=	Adjust pkg-config python checks
+SUBST_FILES.python=	configure.ac cmake/FindPython.cmake
+SUBST_SED.python=	-e 's,python3,python-${PYVERSSUFFIX},g'
 .endif
 
 .include "../../graphics/hicolor-icon-theme/buildlink3.mk"