... and use it for PRECOMPILED_HEADER, INCLUDEPATH, and install target
collection, instead of abusing ProFileReader::absoluteFileValues().

specifically, this falls back to a location in the build directory when
the path is relative and the file cannot be found. in qmake, this
somewhat weird behavior ensures that chaining extra compilers actually
works (and also ensures a lot of frustration with non-clean source dirs
...).

this also fixes INSTALLS with .CONFIG no_check_exists.

Task-number: QTCREATORBUG-14848
Change-Id: Iaf9483c0c4586c464bd10a2aea7cbac7e0df1ec5
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>