* CMakeLists.txt: Set `C_VISIBILITY_PRESET` to hidden for non-Windows only.
Windows handles exports with `DLL_EXPORT` defined, and old MinGW
toolchains emit a lot of warnings about visibility attributes not being
supported in this configuration.
* build/unix/configure.raw: Revise visibility attributes checks by compiling a
better test program with `-Werror` in `CFLAGS` so that warnings correctly
indicate unsupported configurations.