Pthread define unification

In cdi we used 2 different PTHREAD defines.
In the context of the new cmake build system I am trying to clean redundancies and make the Defines a a lot less complicated.
This MR is build on the following assumtions:

• ax_pthread is the source for the HAVE_PTHREAD.
• acx_options is the source of HAVE_LIBPTHREAD.
• acx_options uses ax_pthread.

From this I concluded that the use of HAVE_PTHREAD is the one to be removed as it is defined in the ax_pthread and the, for use in cdi, intended one is HAVE_LIBPTHREAD which is defined after the execution of ax_pthread. Should any of these assumptions be wrong, please let me know.