Fix compil issue under MinGW: for an obscure reason the option -std=c++0x creates error in wxWidget code:
some functions used in wxWidgets ( like _strdup and few other) are not defined when this option is set. 
but the equivalent option -std=gnu++0x works fine so it is used under mingw instead of  -std=c++0x.
(Under Linux/OSX  the option -std=c++0x is used)