Use thread_local instead of compiler extensions (#210)

Use thread_local instead of compiler extensions (#210)

C++11 introduced the thread_local keyword, so we don't need to use
non-standard extensions or our custom pthread extension anymore.
The behavior was previously introduced as a workaround in commit
753a072c542c1c254d7c6adbf10e091ba585ede5. This commit
introduces the macro CVC4_THREAD_LOCAL that can be used to
declare variables as thread local. For Swig, this macro is defined to
be empty.