Fixed bug in BUNDLE: it would try to use UPX, even if it wasn't present,...

... for

an OSX windowed build.  This would terminate pyinstaller prematurely.

build.py has been modified so that if UPX is not present, neither the "upx" or "upx_binaries" attributes (as appropriate) get set in the EXE or COLL classes.  This keeps this attribute from propagating the BUNDLE class, since that's where the BUNDLE class inherits the flag from.