QNX: Add support for CMAKE_SYSROOT

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.

1 files changed, 3 insertions, 0 deletions

diff --git a/Help/manual/cmake-toolchains.7.rst b/Help/manual/cmake-toolchains.7.rst
index f233d08..e8badd4 100644
--- a/Help/manual/cmake-toolchains.7.rst
+++ b/Help/manual/cmake-toolchains.7.rst
@@ -233,6 +233,9 @@ value to those supported compilers when compiling:
   set(CMAKE_CXX_COMPILER QCC)
   set(CMAKE_CXX_COMPILER_TARGET ${arch})
 
+  set(CMAKE_SYSROOT $ENV{QNX_TARGET})
+
+
 Cross Compiling for Windows CE
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