firedrake-install should more carefully control compiler environment #1396
Description
We are presently rather ad-hoc when setting environment variables before calling subprocesses (such as pip, or otherwise) which might build things.
We should tidy it up so that the setting happens globally in an appropriate manner.
(Noticed via imperial cluster builds: the PyOP2 setup sets CC=mpicc unless CC is already set, if it happens that CC is not set to an MPI compiler, things break).