[configure] Don't always look for GC_LIBRARY.

We only need GC_LIBRARY set when we are using GC_USE_STATIC_BOEHM,
which is only the case by default on Mac OS X, which means that we
don't need to go through all of this logic on other platforms.
[configure] Reset some values when using MPS.

When using MPS, make sure that GC_LFLAGS, GC_LIBRARY and
GC_USE_STATIC_BOEHM are reset to the correct values.