dev environment setup and usage

I eliminated some warnings;
all those remaining are the override warnings
which can be suppressed with -Wno-inconsistent-missing-override.

Also it turns out there is a button in Qt creator to suppress warnings (like CONFIG += warn_off);
it's the triangle with the exclamation point in the issues tab.

https://github.com/LibreCAD/LibreCAD/issues/656

Did you succeed in building with Qt Creator?
I wrote this more recently:
https://github.com/LibreCAD/LibreCAD/wiki/Becoming-a-developer

The only issue really is downloading xcode and running the commands to accept the license.

I would say it's worth the download, since building LibreCAD with clang takes about
3/8 the amount of time as with gcc; and no extra setup is required with Qt Creator.