Honor $CC/$CC_host and friends in make generator.

pylib/gyp/common.py

Index: pylib/gyp/common.py
diff --git a/pylib/gyp/common.py b/pylib/gyp/common.py
index 5b06a9746fb0097b9f0ff366b4e47c521f1632c1..6144d2fe7af90bd9690bb92bc234c0a825f2060a 100644
--- a/pylib/gyp/common.py
+++ b/pylib/gyp/common.py
@@ -95,6 +95,15 @@ def BuildFile(fully_qualified_target):
   return ParseQualifiedTarget(fully_qualified_target)[0]
 
 
+def GetEnvironFallback(var_list, default):
+  """Look up a key in the environment, with fallback to secondary keys
+  and finally falling back to a default value."""
+  for var in var_list:
+    if var in os.environ:
+      return os.environ[var]
+  return default
+
+
 def QualifiedTarget(build_file, target, toolset):
   # "Qualified" means the file that a target was defined in and the target
   # name, separated by a colon, suffixed by a # and the toolset name: