Update: thread support.

Update: thread support.

Finish wrapping the pthread functions using f_thread.

I have observed some problems under GLIBC where static linking and -pthreads do not appear to work.
This is a problem with GLIBC as far as I can tell and I must ignore it to the extend that I can.
That said, because of the problems I decided to disable static compiling by default.

Either a better libc should be used or the static libraries should be compiled with thread support disabled.
I have not yet decided whether or not I intend to have thread suppot enabled or disabled by default.
The current default as of this commit is enabled.

There are some tweaks to the build settings to make things slightly easier when building with threads disabled vs threads enabled.

Completely separate f_signal from f_thread (where previously f_signal has conditional thread support).

Add a threadsafe option to fl_execute_parameter_t to conditionally use a threadsafe signal handler or not.