stdenv: Add $preFixupPhases to the phases
glib: Add setup-hook.sh for finding gsettings schemas
baobab: Make use of the gsettings schemas setup-hook
Merge #1901: add preFixupPhases to stdenv; for gsettings

Without this stdenv change it seems difficult to fix some glib's gsettings issues,
as the folders in question may (not) be created in installPhase.