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/buildopts.py

@@ -147,7 +147,7 @@ class BuildOptions(object):
         )
 
     def compute_env_for_install_dirs(self, install_dirs, env=None):
-        if env:
+        if env is not None:
             env = env.copy()
         else:
             env = Env()