add dep on concurrent-output, and re-enable -O0

Using the external concurrent-output library lets it be built with -O2 as
is needed to get good runtime memory use.

Enabling -O0 because ghc is using rather a lot more time and memory due to
the new more complex types.

old master branch:

Linking dist/build/propellor-config/propellor-config ...
24.59user 0.97system 0:25.93elapsed 98%CPU (0avgtext+0avgdata 354612maxresident)k
1544inputs+46064outputs (0major+371244minor)pagefaults 0swaps

this branch before -O0:

Linking dist/build/propellor-config/propellor-config ...
25.56user 0.73system 0:26.61elapsed 98%CPU (0avgtext+0avgdata 345348maxresident)k
0inputs+43480outputs (0major+364163minor)pagefaults 0swaps

this branch with -O0:

Linking dist/build/propellor-config/propellor-config ...
11.91user 0.75system 0:12.97elapsed 97%CPU (0avgtext+0avgdata 237472maxresident)k
16inputs+37264outputs (0major+336166minor)pagefaults 0swaps

Above benchmarks are building all source files needed by config-simple.hs.
The story is rather worse for joeyconfig.hs; building it now needs over 500 mb
even with -O0 :-/

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/todo/depend_on_concurrent-output.mdwn b/doc/todo/depend_on_concurrent-output.mdwn
index fdc66b04..a104c82b 100644
--- a/doc/todo/depend_on_concurrent-output.mdwn
+++ b/doc/todo/depend_on_concurrent-output.mdwn
@@ -8,3 +8,6 @@ Once this is done, can switch GHC-Options back to -O0 from -O.
 -O0 is better because ghc takes less memory to build propellor.
 
 [[!tag user/joey]]
+
+> [[done]]. Didn't wait for it to hit stable; cabal will be used to install
+> it.