Add support for building and testing with flambda to the new CI script

If the flambda environment variable is set to true, the feature is
enabled in the build system.

tools/ci-build-new

@@ -173,6 +173,9 @@ git clean -f -d -x
 case $configure in
   unix)
     confoptions="$confoptions -with-debug-runtime"
+    if $flambda; then
+      confoptions="$confoptions -flambda"
+    fi
     eval "./configure -prefix '$instdir' $confoptions"
   ;;
   nt)
@@ -180,6 +183,9 @@ case $configure in
     cp config/s-nt.h config/s.h
     cp config/Makefile.${OCAML_ARCH} config/Makefile
     sed -i 's%RUNTIMED=.\+%RUNTIMED=true%' config/Makefile
+    if $flambda; then
+      sed -i 's%FLAMBDA=.\+%FLAMBDA=true%' config/Makefile
+    fi
   ;;
   *) error "internal error";;
 esac