BaseTools: Robustness for multiple build environments

BaseTools: Robustness for multiple build environments

When building BaseTools, GNUmakefile builds create directory
BaseTools/Source/C/libs/. On the other hand, Makefile builds use libs
as an nmake pseudo target, so it either must NOT exist as a file or
directory, OR it must phony dependency like .PHONY. The latter
solution conflicts with NmakeSubdirs.py. Therefore, I make it go away
in the cleanall target. I could also add it to the clean target, but
it strikes me that cleanall should be more forceful.

Signed-off-by: Burt Silverman <burtms@gmail.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>