Use weak symbols for process_vm_* functions.

This allows DT to run on pre-2.15 glibcs.
Merge pull request #151 from Hello71/weak

Use weak symbols for process_vm_* functions.
Use qmake builtin debug_and_release.
Use qmake builtin debug_and_release.
Remove OS-specific entries from .gitignore.

.gitignore should only store items that are generated by the code in the
repository. If your operating systems generates temporary files, put
those in your .gitignore_global.