Setting the QMAKE_MANIFEST variable doesn't have
any effect for MSVC. This commit fixes that.
If the developer is setting this variable,
he/she will definitely use CONFIG-=embed_manifest_exe
or CONFIG-=embed_manifest_dll at the same time,
so I think there is no need to check this.
Change-Id: Ie32b7e0cded71efcf14bf4c0eecab5ab1944fa2c
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>