Build shared builds for host unless CT_STATIC_TOOLCHAIN.
In all other situations, build statically, as before.
It is necessary that the static/shared-ness of expat matches
that of gettext on Cygwin/MinGW-w64 as they can't be linked
together if they don't match, so we follow the same logic.
Signed-off-by: Ray Donnelly <mingw.android@gmail.com>