pfff: dont compile facebook/ code by default, use make fb or make fb.opt
pfff: dont compile facebook/ code by default, use make fb or make fb.opt

Summary:
Many times I just modify things in the open source part of pfff
and the compilation time is quite long because of all the binaries
under pfff/facebook that the (slow) ocaml linker has to build.
With this patch 'make' does not compile by default facebook code
so the edit-compile-run loop when I modify sgrep/spatch/codemap/codegraph/scheck
is significantly shorter. For instance
touch lang_php/parsing_php/ast_php.ml; time 'make rec && make pfff_test'
is less than 2secs.

Test Plan:
$ touch lang_php/parsing_php/ast_php.ml
$ time 'make rec && make pfff_test'
1.877 secs

$ time make fb
8.200 secs

Reviewers: pieter, julienv, michalburger1, bill

Reviewed By: bill

Differential Revision: https://phabricator.fb.com/D593248

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