Import CCHOME from the environment when building with GCC on Windows

Import CCHOME from the environment when building with GCC on Windows

Using the environment for such things is preferable to always having to
specify a CCHOME= command-line argument, and is akin to setting various
environment variables via vcvars32.bat for VC++ builds.

Also remove a blank line to make it more obvious that copying DLLs to the
t/ folder is part of the test-prep target. (This is one thing which
doesn't work if CCHOME has not been set and MinGW isn't installed in its
default location.)