component/boost-lib: use simpler syntax for $PYTHON

The ${PYTHON:+--with-python=$PYTHON} looks like a buildout substitution and
because of the way slapos.recipe.cmmi replaces @@LOCATION@@ in options it was
not properly escaped when saved in .installed.cfg.
That was probably a bug, but we don't really need such a complex syntax,
because when using [python] section $PYTHON will be set in environment, so
instead of having the fallback, make sure python section was installed and
assume that $PYTHON is in environment.