SConstruct: port to new Mex pseudo-builder.
SConstruct: port to new Mex pseudo-builder.

The new pseudo-builder is actually a couple of months old now, I just never
ported to it because I didn't test it on Windows. Since Windows users will
probably use compile_timestretch.m anyway, it doesn't matter to me right now.

marcecj/mdirac,marcecj/mdirac,marcecj/mdirac