CMake: Fix tuklib_use_system_extensions

CMake: Fix tuklib_use_system_extensions

Revert back to a macro so that list(APPEND CMAKE_REQUIRED_DEFINITIONS)
will affect the calling scope. I had forgotten that while CMake
functions inherit the variables from the parent scope, the changes
to them are local unless using set(... PARENT_SCOPE).

This also means that the commit message in 5bb77d0920dc is wrong. The
commit itself is still fine, making it clearer that -DHAVE_SYS_PARAM_H
is only needed for specific check_c_source_compiles() calls.

Fixes: c1ea7bd0b60eed6ebcdf9a713ca69034f6f07179