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QCC is a wrapper around GCC, but it is not a fully transparent wrapper. Some compile options need to be passed to GCC using a `-Wc` option. QCC does not support --sysroot, so setting CMAKE_SYSROOT in a toolchain file currently does not work. This means that it is likely that no one is setting CMAKE_SYSROOT in existing QNC toolchain files. Override the GCC option for sysroot in the QCC.cmake file with -Wc,-isysroot. This exposes a further issue in that the QNX SDK does not follow the same architectural folder structure as linux uses. That is, on linux systems, architecture-specific libraries might be in <sysroot>/usr/lib/<arch> such as /usr/lib/x86_64-linux-gnu/libcurl.so CMake models this by suffixing the <arch> onto lib directories when searching for libraries. The QNX SDK is structured differently such that the <arch> should be used as a prefix: <sysroot>/<arch>/usr/lib such as <sysroot>/x86_64/usr/lib/libcurl.so Add a variable for platform configuration to set whether to prefix or suffix the <arch> and set that in the QCC.cmake. Use the directory structure of the QNX SDK to compute the <arch> from the implicit library directories. The assumption is that the arch will be a single directory directly below the CMAKE_SYSROOT, below which the usr/ prefix occurs. It would not be appropriate to instruct users to make the <arch> part of the sysroot when specified in the toolchain file because: 1. That would be non-DRY - The QCC wrapper already determines the <arch> by the -V argument passed to the compiler, specified in the toolchain file as the CMAKE_C_COMPILER_TARGET variable. 2. The includes in the QNX SDK are not below the <arch> directory. So, the location of the <arch> in the full path is different on QNX compared to, say an embedded linux platform, but the intent is the same. Add documentation to recommend the use of CMAKE_SYSROOT in a QNX toolchain file. As the CMAKE_SYSROOT is always the same for QNX, it would be possible to simply set it in QCC.cmake. However, that would change behavior for existing users as when CMAKE_SYSROOT is set, files/paths outside of the CMAKE_SYSROOT do not get found. The <arch> prefixing is only enabled in cmSearchPath.cxx if CMAKE_SYSROOT is set. This ensures that the user gets consistency in the current state without CMAKE_SYSROOT, and gets better consistency when using CMAKE_SYSROOT.
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