Compiling qdoc fails due to missing stdlib.h

Building dev I found the following problem (https://pastebin.com/q7xK8EQT)

> make[3]: Entering directory '/home/crmaurei/dev/qtdev-install/qttools/src/qdoc'
> .....
> fatal error: stdlib.h: No such file or directory

The problem seems to be that the argument of the `-isystem` parameter is `/usr/include` and then, is included after all the other paths.
A simple fix would be to replace it by a `-I`, but maybe is worth to look at:
qttools/mkspecs/features/qt_find_clang.prf since the responsible variable is CLANG_INCLUDEPATH in `qttools/src/qdoc/qdoc.pro`

Official gcc bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129
(won't be fixed, because it depends on the user proper configuration)