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.