Rework to reuse pthread pool code already in dispatch

Author: dgrove-oss

configure.ac

@@ -323,7 +323,7 @@ AC_CHECK_HEADERS([pthread_machdep.h pthread/qos.h])
 # Look for own version first, then for libpwq in our source tree, finally system version.
 AC_ARG_ENABLE([internal-libpwq],
   [AS_HELP_STRING([--enable-internal-libpwq],
-    [Use libdispatch's own implementation of pthread_workqueue API.])],,
+    [Use libdispatch's own implementation of pthread workqueues.])],,
   [case $target_os in
       linux*)
         enable_internal_libpwq=yes