This is what I used to work around the python 3.13 problem on ports-gcc
archs (or actually archs using gcc4 in base that lacks atomic
instructions).

py-gobject3 needs to set COMPILER and LANGS early because of all the
module loading. qt5 and qt6 have to set COMPILER_LANGS since code includes
lang/python as module and that would then downgrade the c++ compiler to
base-gcc :(

I did not dig through all failures yet, maybe there are a few more but
even then I think this is the right step for now.
-- 
:wq Claudio

Index: devel/py-gobject3/Makefile
===================================================================
RCS file: /cvs/ports/devel/py-gobject3/Makefile,v
diff -u -p -r1.99 Makefile
--- devel/py-gobject3/Makefile	10 Nov 2025 05:28:43 -0000	1.99
+++ devel/py-gobject3/Makefile	20 Nov 2025 14:32:27 -0000
@@ -17,6 +17,9 @@ PERMIT_PACKAGE=		Yes
 
 WANTLIB += cairo cairo-gobject ffi girepository-2.0 glib-2.0 gobject-2.0
 
+COMPILER=		base-clang ports-gcc
+COMPILER_LANGS=		c
+
 MODULES=		devel/meson \
 			lang/python \
 			x11/gnome
Index: lang/python/3/python.port.mk
===================================================================
RCS file: /cvs/ports/lang/python/3/python.port.mk,v
diff -u -p -r1.11 python.port.mk
--- lang/python/3/python.port.mk	14 Nov 2025 08:20:58 -0000	1.11
+++ lang/python/3/python.port.mk	20 Nov 2025 13:56:01 -0000
@@ -9,6 +9,9 @@ CATEGORIES +=		lang/python
 # must be REVISION-bumped.
 # - Keep xenocara/share/mk/bsd.xorg.mk PYTHON_VERSION in sync.
 
+COMPILER ?=		base-clang ports-gcc
+COMPILER_LANGS ?=	c
+
 MODPY_WANTLIB =		python${MODPY_VERSION}
 MODPY_RUN_DEPENDS =	lang/python/3
 MODPY_LIB_DEPENDS =	lang/python/3
Index: x11/qt5/qt5.port.mk
===================================================================
RCS file: /cvs/ports/x11/qt5/qt5.port.mk,v
diff -u -p -r1.33 qt5.port.mk
--- x11/qt5/qt5.port.mk	23 Mar 2024 06:54:27 -0000	1.33
+++ x11/qt5/qt5.port.mk	20 Nov 2025 22:05:45 -0000
@@ -174,6 +174,7 @@ MAKE_FLAGS +=	${_MODQT5_SETUP}
 MODQT5_USE_CXX11 ?=	Yes
 .if ${MODQT5_USE_CXX11:L} == "yes"
 COMPILER ?= base-clang ports-gcc
+COMPILER_LANGS = c c++
 ONLY_FOR_ARCHS ?= ${CXX11_ARCHS}
 # useful?
 _MODQT5_SETUP +=	CC=${CC} CXX=${CXX}
Index: x11/qt6/qt6.port.mk
===================================================================
RCS file: /cvs/ports/x11/qt6/qt6.port.mk,v
diff -u -p -r1.13 qt6.port.mk
--- x11/qt6/qt6.port.mk	10 Aug 2025 05:30:00 -0000	1.13
+++ x11/qt6/qt6.port.mk	20 Nov 2025 22:01:59 -0000
@@ -58,6 +58,7 @@ MODQT6_USE_CXX17 ?=	Yes
 
 .if ${MODQT6_USE_CXX17:L} == "yes"
 COMPILER ?= base-clang ports-gcc
+COMPILER_LANGS ?= c c++
 ONLY_FOR_ARCHS ?= ${CXX11_ARCHS}
 .endif