Write build shell environment to a log file

[LourensVeen]

Is your feature request related to a problem? Please describe.
Environment variables like CC or MPICXX influence the build system. If a user has them set incorrectly, then that can cause the build to fail, and it's often difficult to understand why.

Describe the solution you'd like
If we logged the build environment to a file, then we could ask the user to share that and see what's going on. We should be careful though, because environment variables are sometimes used for secrets. So we should probably not log everything, but a list of variables actually used by the build system. Perhaps CC CXX FC F77 MPICC MPICXX MPIFC CFLAGS CXXFLAGS CPPFLAGS LDFLAGS LIBS OMPI_CC OMPI_CXX OMPI_FC OMPI_CPPFLAGS OMPI_LDFLAGS OMPI_LIBS is a good list to start with.