Ovgraphbuild compilation issue with gcc-6

[loic-couderc]

With the issue #44 we noticed that ovgraphbuild do not compile with gcc-6.
This issue seems to come from the way the flags are set in CMakeLists.txt.
More precisely:

set(ENABLE_CXXFLAGS_TO_CHECK 
    -std=gnu++1z 
    -std=c++1z
    -std=gnu++14 
    -std=c++14
    -std=gnu++1y 
-std=c++1y)

Update this file to allow the compilation with gcc-6.