Allow building simbody as cmake subproject

[sradler]

This change is 

[halleysfifthinc]

When Simbody is a subproject, CMAKE_SOURCE_DIR is the directory of the top-level project, and paths in simbody's CMakeLists.txt (e.g. on lines 669, 700, and 717) that assume CMAKE_SOURCE_DIR is the simbody source directory will be incorrect.

[sherm1]

@halleysfifthinc can you confirm that this PR is sufficient to address the problem? If so I'll merge it.

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @sradler)

[halleysfifthinc]

This allows Simbody to build correctly as a subproject, but there are still some other changes (global CACHE variables described in #789) that are needed to make Simbody play nice as a subproject. I think those other changes would best be addressed in a separate PR.