windows build: do not export externals from static library

If you are building a DLL, then you need to explicitly export each
entry point. When building a static library, you should not.

libssh2 was exporting the entry points whether it was building a DLL or a
static library. To elaborate further, if libssh2 was used as a static
library, which was being linked into a DLL, the libssh2 API would be
exported from that separate DLL.
wincng.c: specify the required libraries for dependencies using MSVC

Initially reported by Bob Kast as "for MS VS builds, specify the
libraries that are required so they don't need to go into all
project files that may use this library". Thanks a lot.