Fix dmd source links for DDoc and DDox

[wilzbach]

For Ddoc we have the problem that ddoc is run in the src directory, which means the SRCFILENAME starts with ddmd/...
For druntime this worked before as ddoc is run outside of the src directory.
My simple solution is to add a PROJECT_SOURCE_DIR variable to the project.ddoc file at dmd.

@RazvanN7 as you would need to update these files anyways, I was nice and already set them to dmd (nobody uses the online documentation of dmd at the moment anyways).

[wilzbach]

My simple solution is to add a PROJECT_SOURCE_DIR variable to the project.ddoc file at dmd.

-> dlang/dmd#6591

[rainers]

According to the preview, this doesn't work correctly (anymore?), e.g. ddmd.access links to https://github.com/dlang/dmd/blob/master/src/dmd/access.d. Maybe PROJECT_SOURCE_DIR needs to be updated again?

Also, a lot of the backend source file links look very broken, e.g. http://dtest.dlang.io/artifact/website-4edd0524db57eff02e7186756f620f099ce55137-c3942c8fa8aa4fd3efeed3fda27b4068/web/phobos-prerelease/ddmd_backend_cc.html

[wilzbach]

According to the preview, this doesn't work correctly (anymore?)

I think I have submitted the PR before dlang/dmd#6591 was merged, so the links never worked on DAutoTest.
I just rebased, s.t. DAutoTest can test it again.

[wilzbach]

I think I have submitted the PR before dlang/dmd#6591 was merged, so the links never worked on DAutoTest.
I just rebased, s.t. DAutoTest can test it again.

Yep it worked well. DAutoTest nows shows only the fixed links.
I am going to merge this now as it's a very simple, approved PR that fixes broken source links, DAutoTest likes it and I intend to write a small blog post on the NG about docs for DMD as a library.