Include ut0auxconf.h only if none of the macros it would define is defined.
The check when to include this header was outdated from the time when there
was only one macro involved.

Move the atomics checks that are in univ.i outside of
#if windows ... #else ... #endif
This simplifies the code and removes some duplicates like defining
HAVE_ATOMIC_BUILTINS if HAVE_WINDOWS_ATOMICS is defined in both branches.

Do not define the same macro HAVE_ATOMIC_PTHREAD_T for different events.
Instead define HAVE_IB_ATOMIC_PTHREAD_T_GCC and
HAVE_IB_ATOMIC_PTHREAD_T_SOLARIS.