initialize Babel languages and provide possibility to load extra packages for testing
* default.mk: Run emacs without a site-file. BTEST_EXTRA
redefined to hold names of extra packages to require for
testing. BTEST_PRE and BTEST_POST allow additional
options to be injected around setup of standard load path
for testing. BTEST_OB_LANGUAGES holds the names of Babel
languages to required. R is not added to the list since
it requires ess to be installed and configured. All
variables can be overridden in local.mk as before.