PYTHON2_CMD expansion not quite right

As discussed on #exaile (rel. to bzr rev 3800)

There are a few things wrong with the way PYTHON2_CMD is passed around. I discovered these by changing the PYTHON2_CMD path in Makefile and then trying to install, and it wasn't used in a few places and the resulting 'exaile' script installed in /u/l/b was missing the command completely:

1) there needs to be an export PYTHON2_CMD in the top level Makefile, otherwise it won't get passed to submakefiles or scripts (it will if you pass a PYTHON2_CMD on the make command line, but it won't pass the default set in the Makefile).

2) tools/generate-launcher has the line PYTHON2_CMD?=python2 I'm not convinced that's actually sh syntax, just removing the line and doing (1) makes things work. It wouldn't be a bad idea to guard against an empty PYTHON2_CMD becaus the result is damn confusing.

Dave