the threading libraries is built.  This simplifies the
logic in makefiles that need to check if the pthreads
support is present.  It also fixes a bug where we would
build a threading library that we shouldn't have built:
for example, building with WITHOUT_LIBTHR and the default
value of DEFAULT_THREADING_LIB (libthr) would mistakenly
build the libthr library, but not install it.

Approved by:	re (kensmith)