build: Make life easier with autogen by adding args if none are provided

This patch does 3 things:
1. Fix the --with-enca (which should have been --enable-enca and was failing)
2. If JHBUILD_PREFIX is set, use that when no args are given
3. If no args are given, use the make distcheck default args in place

This will ensure documentation and unit tests are build most of the time and
things don't break so often :)