Add option for link concurrency.

pylib/gyp/generator/ninja.py

Index: pylib/gyp/generator/ninja.py
diff --git a/pylib/gyp/generator/ninja.py b/pylib/gyp/generator/ninja.py
index e3fafb5d43ef61019dbb318d1c867a4aa93cc1ca..b64e7e1499ffba3bdf3b9cd18ddc61397b3b556f 100644
--- a/pylib/gyp/generator/ninja.py
+++ b/pylib/gyp/generator/ninja.py
@@ -1582,6 +1582,10 @@ def CommandWithWrapper(cmd, wrappers, prog):
 
 def GetDefaultConcurrentLinks():
   """Returns a best-guess for a number of concurrent links."""
+  pool_size = os.getenv('GYP_LINK_CONCURRENCY', 0)

[scottmg, 2014/06/05 21:44:53]

int(os.getenv(...))

would be better, otherwise set GYP_LINK_CONCURRENCY=0 will not work as expected.

os.environ.get('GYP_LINK_CONCURRENCY', 0) is generally preferred too, though I
see os.getenv below.

[mtklein, 2014/06/05 21:59:39]

Ah, I see.  PTAL?

This version works I think more as you'd expect: 
(unset)                --> default
GYP_LINK_CONCURRENCY=  --> default
GYP_LINK_CONCURRENCY=0 --> 0
GYP_LINK_CONCURRENCY=N --> N

+  if pool_size != 0:
+    return pool_size
+
   if sys.platform in ('win32', 'cygwin'):
     import ctypes