Configure - Get rid of the special thread_cflag, replace with thread_scheme

The thread_cflag setting filled a double role, as kinda sorta an
indicator of thread scheme, and as cflags.  Some configs also added
lflags and ex_libs for multithreading regardless of if threading would
be enabled or not.

Instead of this, add threading cflags among in the cflag setting,
threading lflags in the lflag setting and so on if and only if threads
are enabled (which they are by default).

Also, for configs where there are no special cflags for threading (the
VMS configs are of that kind), this makes it possible to still clearly
mention what thread scheme is used.

The exact value of thread scheme is currently ignored except when it's
"(unknown)", and thereby only serves as a flag to tell if we know how
to build for multi-threading in a particular config.  Yet, the
currently used values are "(unknown)", "pthreads", "uithreads" (a.k.a
solaris threads) and "winthreads".

Reviewed-by: Andy Polyakov <appro@openssl.org>