win32: Added WinCNG targets to generated Visual Studio project

Inspired by Bob Kast's reports, this commit enables the compilation
of libssh2 with WinCNG using the generated Visual Studio project files.
This commit adds WinCNG support to parts of the existing Win32 build
infrastructure, until new build systems, like pre-defined VS project
files or CMake files may be added.

This commit and b20bfeb3e519119a48509a1099c06d65aa7da1d7 raise one
question: How to handle build systems, like VS project files, that
need to include all source files regardless of the desired target,
including all supported crypto backends? For now the mentioned commit
added a check for LIBSSH2_OPENSSL to openssl.c and with this commit
the supported crypto backends are hardcoded within Makefile.am.