Fix Makefile rebuilding of object files
Fix Makefile rebuilding of object files

Summary:
I was having problems with `make` not picking up changes to `.c` files. I think the problem is that `build-flow` depends on `build-flow-native-deps` which does not actually depend on the `.c` files, so that step was getting skipped.

As I started tweaking stuff, I eventually just replaced the target that runs `ocamlbuild` on all `.o` files with a rule that directly runs `ocamlopt` on each `.c` file, letting `make` instead of `ocamlbuild` decide when it's dirty. this is faster.
Closes https://github.com/facebook/flow/pull/1753

Reviewed By: gabelevi

Differential Revision: D3270015

Pulled By: mroch

fb-gh-sync-id: 3515c883a1f408b9807a7446b237b8a01415ea74
fbshipit-source-id: 3515c883a1f408b9807a7446b237b8a01415ea74

gabro/flow,chenglou/flow,mroch/flow,nmote/flow,gabro/flow,jeffmo/flow,claudiopro/flow,cesarandreu/flow,JonathanUsername/flow,gabelevi/flow,AgentME/flow,fletcherw/flow,jeffmo/flow,AprilArcus/flow,samwgoldman/flow,jamesgpearce/flow,JonathanUsername/flow,samwgoldman/flow,jdelStrother/flow,ForbesLindesay/flow,jeffmo/flow,samwgoldman/flow,TiddoLangerak/flow,chenglou/flow,jgrund/flow,claudiopro/flow,jgrund/flow,facebook/flow,MichaelDeBoey/flow,mroch/flow,popham/flow,JonathanUsername/flow,ylu1317/flow,facebook/flow,JonathanUsername/flow,ForbesLindesay/flow,gabro/flow,nmote/flow,ForbesLindesay/flow,jamesgpearce/flow,TiddoLangerak/flow,MichaelDeBoey/flow,claudiopro/flow,jamesgpearce/flow,samwgoldman/flow,claudiopro/flow,MichaelDeBoey/flow,jamesgpearce/flow,gabro/flow,gabelevi/flow,jeffmo/flow,MichaelDeBoey/flow,gabelevi/flow,facebook/flow,MichaelDeBoey/flow,gabro/flow,nmote/flow,facebook/flow,jdelStrother/flow,gabelevi/flow,JonathanUsername/flow,TiddoLangerak/flow,ForbesLindesay/flow,ylu1317/flow,nmote/flow,mroch/flow,mroch/flow,gabelevi/flow,facebook/flow,chenglou/flow,facebook/flow,jgrund/flow,jamesgpearce/flow,AgentME/flow,mroch/flow,TiddoLangerak/flow,gabelevi/flow,jgrund/flow,ylu1317/flow,AprilArcus/flow,jeffmo/flow,jgrund/flow,cesarandreu/flow,fletcherw/flow,popham/flow,nmote/flow,chenglou/flow,popham/flow,gabro/flow,ForbesLindesay/flow,TiddoLangerak/flow,jdelStrother/flow,samwgoldman/flow,samwgoldman/flow,chenglou/flow,jamesgpearce/flow,ylu1317/flow,nmote/flow,mroch/flow,chenglou/flow,AgentME/flow,jdelStrother/flow,ylu1317/flow,samwgoldman/flow,facebook/flow,jdelStrother/flow,jamesgpearce/flow,fletcherw/flow,AgentME/flow,JonathanUsername/flow,gabelevi/flow,popham/flow,AprilArcus/flow,jdelStrother/flow,popham/flow,popham/flow,claudiopro/flow,JonathanUsername/flow,nmote/flow,AprilArcus/flow,ForbesLindesay/flow,fletcherw/flow,AprilArcus/flow,popham/flow,AprilArcus/flow,cesarandreu/flow,jeffmo/flow,mroch/flow,AgentME/flow,jgrund/flow,fletcherw/flow,fletcherw/flow,cesarandreu/flow