configure: Don't use symlinks for creating the out of tree makefile

On some platforms (such as msys), symlinks are (poorly) emulated
by simply creating a copy of the file.

This means that when building out of tree, the build tree gets
a copy of the original makefile, which can lead to unintuitive
build errors when the original makefile gets updated later.

Instead simply create a stub makefile which includes the real
one.

Signed-off-by: Martin Storsjö <martin@martin.st>