Extend existing picky compiler options with ones missing compared to
autotools builds. Also sync options between clang and gcc.
Redesign the way we enable these options to avoid the slow option
detection almost completely.
This reduces the number of detections from 35 to zero for clang and
3 for gcc, even after adding a bunch of new options.
clang 3.0 (2011-11-29) and gcc 2.95 (1999-07-31) now required.
Also show enabled picky options.
Ref: https://github.com/libssh2/libssh2/pull/952
Reviewed-by: Daniel Stenberg
Closes#10973
