Qt Creator is a Cross-Platform Qt IDE.

The goal of Qt Creator is to provide a cross-platform, complete Integrated
Development Environment (IDE) to develop Qt projects.

NOTE: QmlDesigner needs Qt Private Headers. If you have intent to build the 
QmlDesigner you need to specify the Qt source location. Besides, It is 
required to apply all patches from Slackware tree and run configure.
Note that configure process takes some time to finish due qmake compile 
process.

Follow these steps to prepare source tree:
1 - Copy Qt SlackBuild from Slackware tree to /usr/src/;
2 - Edit qt.SlackBuild and place a EXIT command AFTER configure command;
3 - Execute qt.SlackBuild;
4 - move /tmp/qt-everywhere-opensource-src-x.x.x to /usr/src

QTSOURCEDIR is set to /usr/src/qt-everywhere-opensource-src-x.x.x 
(Qt version from Slackware tree).