use CC env var on openbsd too

build-support/factor.sh

@@ -96,7 +96,7 @@ set_md5sum() {
 
 set_gcc() {
     case $OS in
-        openbsd) ensure_program_installed egcc; CC=egcc;;
+        openbsd) ensure_program_installed egcc; CC=${CC:=egcc};;
         *) CC=${CC:=gcc};;
     esac
 }