Cleanup BCC makefile (remove absolute GC paths, fix del cmd, update clean)

Cleanup BCC makefile (remove absolute GC paths, fix del cmd, update clean)
(a cherry-pick of commit c9569216 from 'release-7_6')

* BCC_MAKEFILE (gcinclude1): Use relative path.
* BCC_MAKEFILE (gcinclude2): Remove.
* BCC_MAKEFILE (gc.lib, clean): Ignore errors of "del" command.
* BCC_MAKEFILE (gc_cpp.cpp, scratch): Remove rule.
* BCC_MAKEFILE (.rc.res, gctest.exe, cord\de.exe, test_cpp.exe): Use
tabs consistently.
* BCC_MAKEFILE (clean): Remove also *.log, *.tds, "gc.#0*" files, and
the ones which were specified for scratch rule.