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.
refs: document _next_name()

If it's not documented, it doesn't show up in the docs (and we really
should document, anyway).
ciscript: don't use an empty string as a number

An empty string is not a valid number, and some shells complain.

Check instead if $COVERITY is non-empty, which is a common convention
and what we're doing anyway.
cygwin also doesn't have qsort_r
Merge pull request #2315 from jacquesg/qsort-cygwin

cygwin also doesn't have qsort_r
Merge pull request #2314 from libgit2/cmn/known-libs

Assorted CMake fixups