don't turn = into += in cumulative mode

it leads to pathological cases where the number of loop iterations may
go way beyond the reasonable.

this means that users need to avoid using the = operator in alternative
branches that lead to different sources/subdirectories being included
into the project. this is a bit of a corner case anyway, as people
usually add directly to SOURCES/SUBDIRS.

Task-number: QTCREATORBUG-1595
Change-Id: I7783e318fbc2790f6a853ba4e3f4a12db881feb5
Reviewed-by: Daniel Teske <daniel.teske@digia.com>