Symbian: Paths inside the MMP file (sourcepath, and DEF files) are relative to the .pro file, rather than relative to the build tree

Paths inside the MMP file (sourcepath, and DEF files) are relative to the .pro file, rather than relative to the build tree (except for paths to the tmp directory, which are accidentially correct (but no files will be generated there because of the problem above), and SYSTEMINCLUDE, which are again accidentially correct)