Looks like for my configuration one of the system header automatically includes <cmath>. As a quick fix you can add '#include <cmath>' at the top of the file.
Give a man a game engine and he delivers a game. Teach a man how to make a game engine and he never delivers anything.
error: Qt3DExtras/QForwardRenderer: No such file or directory 14 | #include <Qt3DExtras/QForwardRenderer>
??? ... O Crap ... on any ubuntu 20.04 based system the 3DExtras packaged is supplied without its headers (or something to that effect). (due to some bureaucratic issue it seems)
Finally got Resource tool to run (... and to actually export) on: Virtualbox >> Ubuntu 18 >> Qt Creator >> "and lots of searching, reading and apt install's" The lack of having to worry about screwing up my main system definitely helped.
Now to build ... something I could use outside that virtualbox setup. (... no luck there, to much 'unknown to me' stuff to resolve.)