Don't let pkg-config add system lib dirs to the search path

[jonhoo]

In its default configuration, pkg-config adds system-wide library
directories to the linker search path (rust-lang/pkg-config-rs#11).
This causes these directories to be searched before other paths added
by later crates or by -Clink-arg in rustflags. If a library is present in
the system-wide directory and a later build step wants to specifically
link against a different version of that library in another path, the linker
will choose the library from the first search directory it finds. If the linker
doesn't find a library in any of the specified search directories, it falls
back on system-wide paths, so we don't need to print the
path we found libssh2 in if it is in one of those system paths.

See rust-lang/libz-sys#50 for the same fix to libz that landed a while back.
See also alexcrichton/curl-rust#441 for the same fix to libcurl-sys.

[ehuss]

@jonhoo I think this was accidentally reverted in #839. Is that a problem for you?

[jonhoo]

@ehuss Interesting. I haven't experience breakage as a result at least, but I definitely do think the change should be re-introduced since it's the more correct behavior.