Instead of using the C++-linker, use the D compiler to link LDC2 and LDMD

[JohanEngelen]

This removes the need for the CMake logic to figure out what linker flags to pass the C++linker to link D code (50 lines of flaky cmake script).

This works on my Mac. Let's see what Travis and AppVeyor think of it.

[JohanEngelen]

Fixes #1278

[kinke]

For AppVeyor, it somehow uses DMD's link.exe, although DMD's bin directory is the last entry in the PATH environment variable. In #1354, DMD isn't in PATH at all anymore.

[JohanEngelen]

I will quit the work on this.
I have not found a solution for making this work on Windows. DMD cannot link a D program when "main()" is in C++, because it will not link with phobos/druntime and will not add a few exception handling symbols. It then degrades into the same problem we have now where we have to manually specify these things on the commandline.