This field is used by the generic project manager which passes the
".config" file in it. The advantage is that both the SnapshotUpdater and
the clang code model do not need to do anything smart, but can pass it
directly to the preprocessor.

Task-number: QTCREATORBUG-11390
Change-Id: I44fc7b20afd28fb59608412f2cce86af6f7e7d6b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>