meson: fix TLS support under mingw

GCC on Windows complains that "__declspec(thread)" doesn't work, but still
compiles it, so the meson check doesn't work. The warning printed by gcc:
"warning: 'thread' attribute directive ignored [-Wattributes]"

Pass -Werror=attributes to make the check fail instead.

This fixes the test suite (minus gtk tests) on Windows with mingw.