c++ – cannot compile on mint 20.1: stdlib.h: No such file or directory

I am running a fresh install of Linux Mint 20.1 and I’n trying to compile a program for a GPS tracker, but it won’t compile:

In file included from /usr/include/c++/9/bits/stl_algo.h:59,
                 from /usr/include/c++/9/algorithm:62,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/qglobal.h:142,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QtCore:4,
                 from pch.h:27:
/usr/include/c++/9/cstdlib:75:15: fatal error: stdlib.h: No such file or directory
   75 | #include_next <stdlib.h>
      |               ^~~~~~~~~~
compilation terminated.

I have installed a number of additional libraries (libusb, libmarble and a few others) and qmake.

Of course, stdlib.h is present on the system:

$ find /usr -name stdlib.h
/usr/include/x86_64-linux-gnu/bits/stdlib.h
/usr/include/bsd/stdlib.h
/usr/include/stdlib.h
/usr/include/c++/9/tr1/stdlib.h
/usr/include/c++/9/stdlib.h
/usr/include/tcl8.6/tcl-private/compat/stdlib.h

After looking at the other questions about this, I re-installed build-essential, which didn’t help.

I re-installed the g++ compiler, no luck either.

I copied /usr/include/c++/9/stdlib.h to /usr/local/include, but it still complains about a missing stdlib.h

I changed the

#include_next <stdlib.h>

in cstdlib into

#include <stdlib.h>

only to find that the next include cannot find stdlib.h; changing that one produced the third include and so on. So that does not work either.

The program compiles and works on Slackware 14.2 by the way.

What am I missing?