Avoid touching source files in profile-build
This refines the profile-build target to avoid 'touch'ing the sources,
keeping meaningful modification dates and avoiding editor warnings like vi's:
WARNING: The file has been changed since reading it!!!
Do you really want to write to it (y/n)?
Instead of touching sources, the (instrumented) object files are removed,
which has the same effect of rebuilding them in the next step.
As a side effect, this simplifies the Makefile a bit.
No functional change.