have CMakeBuilder emit a script to allow invoking CMake manually

Summary:
While developing on a project it is often convenient to be able to invoke its
build manually, rather than always needing to re-run `getdeps.py`.  This
updates the CMakeBuilder to also emit a script that can be used to manually
run CMake outside of `getdeps.py`.

The CMakeBuilder is the only builder that this really matters for right now,
as pretty much all of the projects where we do first-party development use
CMake for their build system.

Reviewed By: pkaush

Differential Revision: D16477399

fbshipit-source-id: c8a14af158af7b32d6c799ef685b037e68b748ff

build/fbcode_builder/getdeps/envfuncs.py

@@ -16,7 +16,10 @@ import sys
 class Env(object):
     def __init__(self, src=None):
         self._dict = {}
-        self.update(src or os.environ)
+        if src is None:
+            self.update(os.environ)
+        else:
+            self.update(src)
 
     def update(self, src):
         for k, v in src.items():