build: moshfy gen-compiler script
cygwin: There is no __CYGWIN64__!

(See Cygwin FAQ)

Fix cygwin64 build by:

 - Drop direct-threaded(USE_DIRECT_THREADED_CODE)
 - Drop 32bit wchar(USE_16BIT_WCHAR)
mona: dont_free is only for Mona port

- Remove unused functions prototype:
  my_realloc
  my_dont_free
bignum: Remove redundant include <gmp.h> from scheme.cpp
bignum: Add bignum_shim.h
cmake win32: Move GMP check before include file check

So we don't waste time for invalid GMP_DIR settings
cmake: Add USE_MP_GMP flag