Tests/InterfaceBuildTargets: Fix testlib file name

On Borland and Watcom we expect the .lib extension, so add it.
Makefile: Always create clean target command

Borland Make complains if the phony clean target has no rule to build it.
If there are no files to clean, generate and run an empty clean script.
Merge topic 'INTERFACE_LIBRARY-build-targets' into next

28daf50 Makefile: Always create clean target command
0a4c6a4 Tests/InterfaceBuildTargets: Fix testlib file name