From: Jakub Jirutka <jakub@jirutka.cz>
Date: Mon, 3 Apr 2017 20:49:00 +0200
Subject: [PATCH] Add support for global r/o cache of prebuilt libraries

--- a/tools/cache.py
+++ b/tools/cache.py
@@ -29,9 +29,12 @@
 
     if use_subdir:
       if os.environ.get('EMCC_WASM_BACKEND') and os.environ.get('EMCC_WASM_BACKEND') != '0':
-        dirname = os.path.join(dirname, 'wasm')
+        self.subdir = 'wasm'
       else:
-        dirname = os.path.join(dirname, 'asmjs')
+        self.subdir = 'asmjs'
+      dirname = os.path.join(dirname, self.subdir)
+    else:
+      self.subdir = ''
     self.dirname = dirname
     self.debug = debug
 
@@ -90,6 +93,11 @@
       if what is None:
         if shortname.endswith(('.bc', '.so', '.a')): what = 'system library'
         else: what = 'system asset'
+      if shared.GLOBAL_CACHE_DIR and os.getenv('EM_USE_GLOBAL_CACHE') == '1' and not force:
+        path = os.path.join(shared.GLOBAL_CACHE_DIR, self.subdir, shortname)
+        if os.path.exists(path):
+          logging.info('using %s: %s from global cache: %s' % (what, shortname, path))
+          return path
       message = 'generating ' + what + ': ' + shortname + '... (this will be cached in "' + cachename + '" for subsequent builds)'
       logging.info(message)
       self.ensure()