#11 Deactivate GNU extensions compilation

[Sylvain Joubert]

Hi,

Please find attach a patch that changes the C++11 standard required for rvalue support from 'gnu++11' to 'c++11'
There is no technical reason to require the non-standard GNU extensions. However, this lead to some issues for user code.

When activating the GNU extensions, some of the extended features of mathgl will also be activated (e.g. typeof support). Unfortunately, since this configuration is reflected in the config.h file, that means any user of the library will have to compile its own code with the GNU extensions flag. Forcing users to use a non-standard dialect of C++ seems a bad idea in my opinion.

Additionally, I prepended the standard flag instead of appending it. This way one can still compile mathgl with GNU extensions if need be by modifying CMAKE_CXX_FLAGS or CXXFLAGS before configuring.

Thanks