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, 7 insertions, 1 deletions

diff --git a/Source/cmSearchPath.cxx b/Source/cmSearchPath.cxx
index d15ce57..766d347 100644
--- a/Source/cmSearchPath.cxx
+++ b/Source/cmSearchPath.cxx
@@ -181,7 +181,13 @@ void cmSearchPath::AddPrefixPaths(const std::vector<std::string>& paths,
       const char* arch =
         this->FC->Makefile->GetDefinition("CMAKE_LIBRARY_ARCHITECTURE");
       if (arch && *arch) {
-        this->AddPathInternal(dir + subdir + "/" + arch, base);
+        if (this->FC->Makefile->IsDefinitionSet("CMAKE_SYSROOT") &&
+            this->FC->Makefile->IsDefinitionSet(
+              "CMAKE_PREFIX_LIBRARY_ARCHITECTURE")) {
+          this->AddPathInternal(cmStrCat('/', arch, dir, subdir), base);
+        } else {
+          this->AddPathInternal(cmStrCat(dir, subdir, '/', arch), base);
+        }
       }
     }
     std::string add = dir + subdir;