GNUmakefile: add -m64 CFLAGS when targeting mingw64, add -m32/-m64 to LDFLAGS
GNUmakefile: add -m64 CFLAGS when targeting mingw64, add -m32/-m64 to LDFLAGS

libssh2 equivalent of curl patch https://github.com/bagder/curl/commit/d21b66835f2af781a3c2a685abc92ef9f0cd86be

This allows to build for the non-default target when using a multi-target mingw distro.
Also bump default OpenSSL dependency path to 1.0.2c.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2