Log Message:
The mysql4-client libraries depend on libpthread.  When building
packages that depend on mysql-client, using buildlink3, and using
GNU PTH, the pthread library never gets pulled in by buildlink.  This
caused the p5-DBD-mysql build to fail, which in turn caused the mytop
build to fail.  The fix is to include mk/pthread.buildlink3.mk in
mysql4-client/buildlink3.mk.

This addresses PR#28162.