fix warning when generating VS 2013 project

/FS forces the compiler to synchronize pdb file writes.
This option is not needed when building with Visual Studio itself.
Still, qmake needs to know it when parsing the compiler flags.

Task-number: QTBUG-36535
Change-Id: Id5b68c4028844e0b95904e08b5121310a4ff13d6
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
(cherry picked from commit qtbase/9b59e51c5064c67c423389c2c884d009903910c7)
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
qmake vcxproj generator: write PrimaryOutputExtension value

Second attempt. MSVCPROJ_TARGET contains the resolved target name,
including version number and target extension.
We're splitting this value into PrimaryOutput and
PrimaryOutputExtension.
PrimaryOutputExtension is only written if it contains a non-default
value.

Task-number: QTBUG-26782
Change-Id: I4b828dc5dd47322f653585aee1a5767f0cf8bd48
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
(cherry picked from commit qtbase/d1e950b919615c1ae754e07899f1b45303c17654)
Reviewed-by: Andy Shaw <andy.shaw@digia.com>