Support CXXFLAGS, LDFLAGS environment variables

[ucbjrl]

In order to run Synopsis vcs with newer versions of gcc, we need to be able to add some flags to the command line. Since this is dependent on the version of gcc installed in the test environment, the easiest way to do this is to inherit build flags (CC, CXX, CCFLAGS, CXXFLAGS, CPPFLAGS, LDFLAGS) from the environment. We had support for this in Chisel2. Should we provide support in chisel3?