matplotlib: fix build on darwin

* build with clangStdenv
* update to 1.2.1
haskell-cmdargs: update to version 0.10.5
haskell-feed: update to version 0.3.9
haskell-wai: jailbreak to fix build with vault 0.3.0.0
haskell-haskell-src-meta: update to version 0.6.0.3
haskell-unix-process-conduit: update to version 0.2.1.2