superenv: filter -Wno-long-double

This flag was supported in gcc-4.0, but not later GCCs or clang.
It pops up in older python-configs, and may also come up in other
contexts.

Fixes mistydemeo/tigerbrew#213.
libssh2: HEAD

Adds a HEAD build to Libssh2. The main release branch hasn't been updated in nearly 2 years, but the git repository gets updated regularly. Feel like the head option could be pretty useful to some consequently.

Closes #31630.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>