Fix gcc 5.4 build

[mlamarre]

This PR adds the CXX switch -std=c++11 in config-linux-gcc.py.

I don't know why I'm the first to hit this, but If I don't specify -std=c++11, I get the same build errors as in this issue: https://github.com/mitsuba-renderer/mitsuba/issues/7

I'm building from a minimal Ubuntu 16.04 docker file. I noticed the debug variant already had the c++11 switch at the end, I moved it to the front. There's an extra -fPIC switch in CXXFLAGS in the debug variant. Removed it and it's still added to every g++ calls so it's unnecessary.

[wjakob]

Seems reasonable -- can you leave the -fPIC flag? I want to ensure that it really gets used for every target (not just shared libraries).

[mlamarre]

I put back fPIC and also add it to the release file.