Disable GCC plugins for out of tree kernel modules

kernel modules built in VM often use different gcc version, which makes
provided gcc plugins incompatible. Since rebuilding those plugins may
not be straight forward (extra gcc headers needed, possibly kernel
config changes needed if gcc version differs significantly), disable
them for out of tree modules.

Fixes QubesOS/qubes-issues#2844

kernel.spec.in

@@ -265,7 +265,11 @@ cp Module.markers %buildroot/lib/modules/%kernelrelease/build
 fi
 
 rm -rf %buildroot/lib/modules/%kernelrelease/build/Documentation
-cp .config %buildroot/lib/modules/%kernelrelease/build
+
+# disable GCC plugins for external modules build, to not fail if different gcc
+# version is used
+sed -e 's/^\(CONFIG_GCC_PLUGIN.*\)=y/# \1 is not set/' .config > \
+        %buildroot/lib/modules/%kernelrelease/build/.config
 
 rm -f %buildroot/lib/modules/%kernelrelease/build/scripts/*.o
 rm -f %buildroot/lib/modules/%kernelrelease/build/scripts/*/*.o