... instead of the import lib.  If the DLL is an auxtarget, it cannot
be deleted without annoying errors, and we want to be able to rm -r
$(INSTIDR) and build incrementally from there.

- replace DLLTARGET -> ILIBTARGET
- replace gb_Library_DLLFILENAMES -> gb_Library_FILENAMES
- replace gb_Library_FILENAMES -> gb_Library_ILIBFILENAMES
- replace gb_Library_get_dllname -> gb_Library_get_filename
- replace gb_Library_get_filename -> gb_Library_get_ilibfilename
- replace gb_CppunitTest_get_libfilename -> gb_CppunitTest_get_ilibfilename
- replace gb_LinkTarget_set_dlltarget -> gb_LinkTarget_set_ilibtarget

Change-Id: I92a2f061a653b9b5941f3232d729987b1317b6f8