changed how building is triggered, now there is a [twn] shell script that is exptected to be in PATH. also physfs and libxm cmake targets no longer recompiled per every target, but shared

tools/twn

@@ -0,0 +1,15 @@
+#!/bin/env sh
+# townengine tooling interface
+
+set +e
+
+toolpath="$(dirname -- "${BASH_SOURCE[0]}")"
+export TWNROOT=$(realpath "$toolpath"/../)
+export TWNBUILDDIR=$(realpath "$toolpath"/../.build)
+
+case "$1" in
+    build ) "$toolpath"/build.sh
+            ;;
+    *     ) echo "Unknown command."
+            ;;
+esac