[Users] Build dependency error corrected

[Erik Schnetter]

I just committed several corrections to a benign build dependency
error that led to unnecessary rebuilds of many source files whenever a
ccl file was modified.

Several thorns in ExternalLibraries create a file called "done" to
indicate that they are finished configuring their external
dependencies, and that other thorns can now build their source files.
In several cases, these "done" files were touched (modified)
unnecessarily when the external dependencies were unchanged. Via a
chain of dependencies, this triggered rebuilds of many source files in
many thorns, in particular Fortran source files.

-erik

-- 
Erik Schnetter <schnetter at gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/