Premake: replaced buildoptions -std=c++11 with flags C++11

[hsandt]

On Linux at least, usage of C++11 is not recognized when premake just uses buildoptions "", causing errors during make such as:

../../Box2D/Collision/Shapes/b2ChainShape.h:99:15: error: ‘nullptr’ was not declared in this scope
  m_vertices = nullptr;

The issue may be related to premake/premake-core#770 for which the solution is to use cppdialect, only available starting premake 5 alpha 12 (currently WIP). For now, I suggest this fix, although redundant and not as elegant as filters.

I also mention the fix on Stack Overflow.

[hsandt]

With alpha12 of premake, I got this:

** Warning: the flags value C++11 has been deprecated and will be removed.
Use cppdialect "C++11" instead
@/.../Box2D-repo/Box2D/premake5.lua(25)

Therefore, I will close this PR and make a new one for alpha12 using cppdialect (I could juste update the PR, but since I made the PR directly on master it may interfere with other changes I make on master, so I prefer using a dedicated branch).

[hsandt]

OK, I realized a bug on premake5-alpha12 makes cppdialect fails on OSX (it writes std=C++11 instead of std=c++11). I will send them a PR, but until then, the flags solution works, even though deprecated. I reopen this PR so that, at least, we have a solution to nullptr and other bugs in the meantime.

[hsandt]

PR to fix cppdialect on OSX on the way: premake/premake-core#897

[hsandt]

PR accepted. Will send another PR with cppdialect.