Describe the bug

According to https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4277 --sysroot is not supported directly by the qcc compiler for QNX and it should be -Wc,-isysroot used instead. But the Autotools Toolchain generator is always adding --sysroot when the tools.build:sysroot config is set. I think a check should be added if the target OS is QNX and use the recommended alternative flag then.

I encountered this issue while trying to compile libbacktrace for QNX and got the following error output: cc: unknown option: '--sysroot'.

How to reproduce it

Build libbacktrace for QNX with "tools.build:sysroot" configuration defined