Fix build issues on MinGW when finding OpenSSL.

* Add FindOpenSSL.cmake from CMake 3.0.2.
* Change MinGW find library to find libssl and libcrypto as well as libeay32 and ssleay32.  Apparently
 at some point OpenSSL changed the library file names for MinGW.
* Stop find_library() from looking in system paths and returning the actual DLLs rather than the link
  libraries.