configure: Add --with-system-bdwgc option

Add an option to enable system-installed BDW-GC as gc implementation.

This option enables to use shared library version of libgc.

Don't forget configuring bdw-gc with --enable-cplusplus. Mosh uses
bdw-gc C++ support.

Caveats:

 - It won't work on Cygwin32. I'm still investigating why.
 - It defines DISABLE_ANN_PAIR_OPTIMIZATION which disables
   annotated-pair optimization.
Cygwin: Consider Cygwin64 build