Fix TLS detection: test with linker, add --disable-compiler-tls

NetBSD 5.1 and older, as well as Darwin, succeed to compile code
containing TLS, but cannot link it. Test with linker in addition to
compiler for TLS support.

Also add a --disable-compiler-tls configure option to allow users to
force using the pthread getspecific fall back.

Fixes #288

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>