games/wxlauncher: unbreak WX3 option on DragonFly and FreeBSD 8.x/9.x

After r383466 building against wxgtk30 only worked with clang/libc++
because libstdc++ 4.2 in base lacks C++11 support while newer libstdc++
(from lang/gcc* or in base on DragonFly) requires explicit -std=c++11.

Also, -std=c++11 on FreeBSD turns NULL into nullptr which sometimes
breaks build.

PR: 199916
Approved by: lightside <lightside@gmx.com> (maintainer)