tools: backport tools/install.py for headers

Backport the tools/install.py changes from 628a3ab that were missed
when 6fb0b92 backported the corresponding changes to the Makefile to
build the headers only archive.

PR-URL: #4149
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>

tools/install.py

@@ -139,6 +139,9 @@ def files(action):
 
   if 'true' == variables.get('node_install_npm'): npm_files(action)
 
+  headers(action)
+
+def headers(action):
   action([
     'common.gypi',
     'config.gypi',
@@ -183,8 +186,13 @@ def run(args):
   if len(args) > 2: dst_dir = abspath(args[2] + '/' + dst_dir)
 
   cmd = args[1] if len(args) > 1 else 'install'
-  if cmd == 'install': return files(install)
-  if cmd == 'uninstall': return files(uninstall)
+  if os.environ.get('HEADERS_ONLY'):
+    if cmd == 'install': return headers(install)
+    if cmd == 'uninstall': return headers(uninstall)
+  else:
+    if cmd == 'install': return files(install)
+    if cmd == 'uninstall': return files(uninstall)
+
   raise RuntimeError('Bad command: %s\n' % cmd)
 
 if __name__ == '__main__':