inkscape: add python to the PATH, fixes #3449
glib-tested: fix running tests
Fix NixOS evaluation on i686-linux
GHC 7.6 and earlier cannot compile aeson 0.8.x.
emacs: Fix building on Darwin with recent versions of GCC

This gets rid of the "unexec: unrecognized section __static_data in
__DATA segment" error while dumping Emacs.
emacs: On Darwin, don't depend on gconf and cairo
GHC 7.6.x required scientific 0.2.x to build aeson 0.7.x.