[CMake] Source file symlinks on out-of-source builds

[Rodolfo Lima]

Hi people, I was thinking if it would be nice if cmake created symlinks
on supported platforms of source files into CMAKE_CURRENT_BINARY_DIR
when it differs from CMAKE_CURRENT_SOURCE_DIR. My only complaint against
out-of-source builds is that the sources aren't around when I do make.
This feature would help a lot.

Since I've got some time to spare, if this shows to be useful to more
people I could try to implement it and forward the patch upstream.

Regards,
Rodolfo Lima