Implement a check whether pthread_t objects can be used by GCC atomic
builtin functions. This check is implemented in plug.in and defines the
macro HAVE_ATOMIC_PTHREAD_T. This macro is checked in univ.i and the
relevant part of the code enabled (the one that uses GCC atomics against
pthread_t objects).

In addition to this, the same program that is compiled as part of the
plug.in check is added in ut/ut0auxconf.c. In the InnoDB Plugin source
archives that are shipped to the users, a generated Makefile.in is added.
That Makefile.in will be modified to compile ut/ut0auxconf.c and define
the macro HAVE_ATOMIC_PTHREAD_T if the compilation succeeds. I.e.
Makefile.in will emulate the work that is done by plug.in. This is done in
order to make the check happen and HAVE_ATOMIC_PTHREAD_T eventually
defined without regenerating MySQL's ./configure from
./storage/innobase/plug.in. The point is not to ask users to install the
autotools and regenerate ./configure.

rb://95

Approved by:	Marko