#13 fix C++14 compilation

[Anonymous]

As C++ 14 makes the GNU Complex Numbers Extension incompatible ( https://gcc.gnu.org/onlinedocs/gcc/Complex.html ) please integrate https://sourceforge.net/p/mathgl/patches/11/ .

Furthermore it's necessary fixing the header mgl2/datac.h at two points (l 413 and 421) where explicit casts from double to (dual) are necessary.

Just test it using a C++14 compiler and -std=c++14 or higher.