Support out-of-source build for qmake (closes: #964214)

This implements handling for out-of-source builds in the make build system. Qmake handles out-of-source builds quite normally, you do them by running qmake in the build dir and pointing qmake to the correct source tree or project file. The only challenge for debhelper is to map the filenames to point to the source filenames if they are relative.

To do it, this patch mimics the parameter handling of qmake and checks which arguments might be files or directories and rewrites them (-set/-query/-unset are handled out of precaution, although they don't work in this context). If no project files or directories are present, a default pointing to the source directory is added. Also, QMAKE_EXTRA_ARGS is preserved verbatim.