Unquiet CMake output

The point of this phase is to know what we have and not. Show the user a
clear indication of what we have.
Workaround missing .pc files on FreeBSD

This fixes #2118
Don't exclude libssh2 from MinGW

It reportedly works. It does not however work when cross-compiling on
Travis, so let's disable it there.

This fixes #2311.
Merge pull request #2314 from libgit2/cmn/known-libs

Assorted CMake fixups