[btls] Pass CC and CXX to the BTLS CMake and disable BTLS on older GCC (#4200)

[btls] Pass CC and CXX to the BTLS CMake and disable BTLS on older GCC (#4200)

CMake doesn't pick up the compiler from the PATH by default,
it only looks in default locations so without this we might
end up using a different compiler for BTLS than for the main
Mono build which we don't want.

BTLS also requires a fairly new GCC which includes C++11
features, added a quick check for stdalign.h which BTLS
requires and disable it if the header isn't found.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=46712