Building MySQL plugin via qmake and nmake (msvc19)

Hey everybody,

I need to reinvite https://bugreports.qt.io/browse/QTBUG-63796

Because the documentation does not work as expected

https://doc.qt.io/qt-5/sql-driver.html#qmysql

Especially if you add source via Qt Maintenance Tool and did not compile the whole git from scratch.

I also needed to mask QMAKE_USE += mysql in the mysql.pro file

and generate the make files with:

qmake INCLUDEPATH+=C:\pathto\mysql\include LIBS+=C:\pathto\mysql\lib\libmariadb.lib mysql.pro

after that nmake and nmake install works nicely.

Using MYSQL_INCDIR and MYSQL_LIBDIR like shown in the documentation does not work if you only try to add the mysql plugin afterwards.