Add missing default to PharData::decompress()

Summary: Argument was supposed to be optional.

Fixes #4264

Reviewed By: @JoelMarcey

Differential Revision: D1687909

Signature: t1:1687909:1416344911:22e759b5c409f884374a5d8f4dedc97e2374f3cc
Make the server tests more robust

Summary: Choose ports multiple times in case of random collisions.
Also share more code between fastcgi and http tests.

Reviewed By: @swtaarrs

Differential Revision: D1691305

Signature: t1:1691305:1416441190:9c1229c04d420b2c9c96714fd12b4c73892fab0e
cmake option to enable avx2 instruction support

Summary: Creating a new pull request for enabling avx2 support in gcc and creating a C/CXX flag which can be used to statically compile avx2 instructions.

I will delete these commits from #4200
Closes https://github.com/facebook/hhvm/pull/4284

Reviewed By: @paulbiss

Differential Revision: D1691802

Signature: t1:1691802:1416442795:e3341b2684b4a72f8f5c0db2645288a0f779e103