pbkdf2-hmac-sha256-opencl: Simplify cmp_all(). We don't need to test more
than 32 bits, the rest are checked in cmp_one() and this will happen very
infrequently.
add documentation for cross-compiling MinGW builds
MinGW portability fixes # 2 (for Makefile)
MinGW portability fixes # 1
GMP is not worth the trouble for MinGW builds (revisit later)