Handling of qmake variable defining the manifest file on MinGW

On Windows, the application manifest file can be linked with the
executable, to specify for example the requested privileges of the
application. On MSVC nmake, the manifest is already handled in
NmakeMakefileGenerator::writeBuildRulesPart, but it is not compatible
with MinGW. On MinGW, this manifest file has to be referenced in the
Rc File. This patch simply handles the existing variable
"QMAKE_MANIFEST" which defines the appropriate line RT_MANIFEST in
the RC file.

Task-number: QTBUG-42454
Change-Id: I921606e002ffe3801c537f30ac2365891f97d5c9
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>