fbcode_builder: getdeps: add envfuncs.py
fbcode_builder: getdeps: add envfuncs.py

Summary:
Add some helpers for manipulating environment variables
that will hold either compiler flags or paths.

These are used in later diffs when setting up arguments/environment
for eg: cmake.

Reviewed By: sinancepel, simpkins

Differential Revision: D14690993

fbshipit-source-id: 7f9753cd99d968550fe9e3ba8b7017a44683061e

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst