win32-msvc2013: force synchronous writes to .pdb files

Synchronize .pdb file writing, otherwise parallel builds
will fail, when different compiler instances try to access
the same .pdb file.

See also
http://msdn.microsoft.com/en-us/library/vstudio/dn502518.aspx

Task-number: QTBUG-36398

Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from commit 59ae8323b7a57cc04d19fc1f552aeaeda9dad3f7)

Change-Id: I7263b875a1015724bae6c37fb540d52507a1c7da
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>