Use a separate build directory for each port

This allows building them simultaneously with Ninja.
Split out from emscripten-core#23924

Author: dschuff

tools/ports/__init__.py

@@ -6,6 +6,7 @@
 import logging
 import hashlib
 import os
+from pathlib import Path
 import shutil
 import glob
 import importlib.util
@@ -176,7 +177,9 @@ def install_headers(src_dir, pattern='*.h', target=None):
 
   @staticmethod
   def build_port(src_dir, output_path, port_name, includes=[], flags=[], cxxflags=[], exclude_files=[], exclude_dirs=[], srcs=[]):  # noqa
-    build_dir = os.path.join(Ports.get_build_dir(), port_name)
+    mangled_name = str(Path(output_path).relative_to(cache.get_sysroot(True))).replace(os.sep, '_').replace('.', '_')
+    build_dir = os.path.join(Ports.get_build_dir(), mangled_name)
+    logger.debug(f'build_port: {port_name} {output_path} in {build_dir}')
     if srcs:
       srcs = [os.path.join(src_dir, s) for s in srcs]
     else: