Rely on ciso646 and __cplusplus macro when detecting cxx, add cxx11 flag

[sovrasov]

It seems that _LIBCPP_VERSION is not defined in the string header at least for GCC 5.4 and above.

printf "#include <string>\nint main () {}" | g++ -E -x c++ -dM - | grep CPP

[sovrasov]

@jschueller please take a look. I need CXX11 macro in #194
It doesn't affect current algorithms, but can be useful in future.

[stevengj]

Is this actually used anywhere? Why do we need -std=c++11 at all?

[sovrasov]

I'm going to use it to enable/disable AGS (like STOGO), since it's code heavily depends on c++11. May be it can be useful for future contributors who also use new C++. Now NLOPT_CXX is used only for STOGO and I thought to join these flags, but we can't assume that everyone who has c++ has c++11 compiler, unfortunately.

[stevengj]

The STOGO code is ancient — mostly written in 1998, and incorporated in NLopt in 2007. How could it possibly depend heavily on C++11?

[sovrasov]

I meant future code and the code from #194.

[sovrasov]

Moved to #194