Workaround for missing updated libgcc.i686

Dnf don't like the same package for different architectures to be in
different version, which prevent installing updated gcc. Force it by
explicitly specifying libgcc.x86_64.

This is ugly, especially the part about exact gcc version, but since
fc25 is EOL, it shouldn't result in any problems...

Makefile.builder

@@ -11,6 +11,9 @@ else ifeq ($(PACKAGE_SET),vm)
   endif
 endif
 
+ifeq ($(DIST),fc25)
+    SOURCE_PREP := workaround-gcc-upgrade-fc25
+endif
 
 source-debian-xen-copy-in: VERSION = $(shell cat $(ORIG_SRC)/version)
 source-debian-xen-copy-in: ORIG_FILE = "$(CHROOT_DIR)/$(DIST_SRC)/xen_$(VERSION).orig.tar.gz"
@@ -19,3 +22,6 @@ source-debian-xen-copy-in:
 	-$(ORIG_SRC)/debian-quilt $(ORIG_SRC)/series-debian-vm.conf $(CHROOT_DIR)/$(DIST_SRC)/debian/patches
 	tar xfz $(SRC_FILE) -C $(CHROOT_DIR)/$(DIST_SRC)/debian-vm --strip-components=1 
 	tar cfz $(ORIG_FILE) --exclude-vcs --exclude=debian -C $(CHROOT_DIR)/$(DIST_SRC)/debian-vm .
+
+workaround-gcc-upgrade-fc25:
+	sudo chroot $(CHROOT_DIR) dnf install -y gcc-6.4.1-1.qubes1.fc25.x86_64 libgcc.x86_64