Fix cmake export to allow cross-compilation with sysroot

[aurelien-enchanted-tools]

Fix cmake export to allow cross-compilation with sysroot

[nim65s]

Hi @aurelien-enchanted-tools,
I would agree that installing in a relative path is better.

But this was added in #179 by @jcarpent to fix windows compilation, I don't exactly understand why, as I have no clue about how windows work.

@jcarpent : is this still relevant ?

[jcarpent]

Please fix the following:

Sorry @aurelien-enchanted-tools, I don't understand your request in such phrasing.
We are first and foremost real humans developing advanced and versatile open-source software.
At minima, a thankful comment would have been appreciated, especially because you're using our tools for developing your product at Enchanted Tools. Don't you think so?

Related to your issue, could you elaborate more on the issue that you currently face with the CMake packaging of hpp-fcl?

[aurelien-enchanted-tools]

Ok. Above, the fix shows how to solve the below issue, when someone tries to import the hpp's cmake export, in the case of cross-compilation with sysroot :

CMake Error at […]/sysroots/armv8a-poky-linux/usr/lib/cmake/hpp-fcl/hpp-fclTargets.cmake:95 (message):
  The imported target "hpp-fcl::hpp-fcl" references the file

     "/usr/lib/libhpp-fcl.so"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "[…]/sysroots/armv8a-poky-linux/usr/lib/cmake/hpp-fcl/hpp-fclTargets.cmake"

  but not all the files it references.

The fix solves the issue and allows building the hpp package as dependencies of another package. When the application or lib uses cmake find_package() in its CMakeList.txt.

The usual way to cross-compile with a sysroot, an application or a lib, is to have these dependencies populated into a target sysroot/staging.
To populate the sysroot with a new lib, the lib has to be cross-compiled to populate the sysroot as:

cmake -G Ninja -DCMAKE_INSTALL_PREFIX="/usr"
DESTDIR="${PATH_TO_THE_SYSROOT}" ninja install

Once done, the new dependence is in the sysroot/staging directory as all other target rootfs dev dependencies (*. so and headers). This allows to have a sysroot to develop others libs or applications, etc.