Import build flags from environment (a la Chisel2).

[ucbjrl]

Import CC, CXX, CCFLAGS, CXXFLAGS, CPPFLAGS, LDFLAGS and incorporate them in the flags used to build C++ simulations.

Although similar effects can be achieved via the --more-vcs-flags, and --more-vcs-c-flags command line options, it's difficult to make these available to the internal ScalaTest suites.

[chick]

If there's some urgency, maybe my comments should be turned into issues for testers2 so that it can be a bit more robust

[ucbjrl]

I think this does need to be discussed. I like the idea of issuing some kind of notification when environment variables are used.