meson: Add feature to disable compiler TLS support

When compiling with MinGW, use of the __thread attribute causes pixman
to gain a dependency on the winpthread DLL.  With Autotools, this could
be avoided by configuring with ac_cv_tls=none, causing pixman to fall
back to TlsSetValue() instead.

Add a Meson 'tls' option that can be 'disabled' to skip support for TLS
compiler attributes, or 'enabled' to require a working TLS attribute.
2 files changed