avoid leaking -std=gnu++11 flag to user's targets

[mean-ui-thread]

This fixes #126

[yse]

Thanks for good advice about private flag with -std=gnu++11. I think it's a very good solution of problem #71.
Can you please add set(CMAKE_CXX_STANDARD 11) in subdirectories (profler_gui, easy_profiler_converter etc) because we need to set the flag explicitly now =)

[mean-ui-thread]

@yse sure. however, adding set(CMAKE_CXX_STANDARD 11) to the root level CMakeLists.txt will also accomplish the same thing with a lot less effort. Please bare in mind that CMAKE_CXX_STANDARD only works from cmake 3.1 and up. older cmake versions would require manually adding -std=gnu++11 like the way you've done already.

[mean-ui-thread]

@yse job is done. This will affect every subdirectories (core, gui, etc) and won't propagate upward for those who are adding easy_profiler as a subdirectory in their project.

[mean-ui-thread]

@yse no longer needed. Taken care of by the root cmakelists.txt now.

[mean-ui-thread]

@yse are you okay with my latest change proposal?

[yse]

Oh, excuse me, I was very busy in other project and missed some letters.
Very good patch. Thanks a lot!