Signed-off-by: Ian Chen <ichen@openrobotics.org>

This PR uses the changes introduced in gz-cmake3 in gazebosim/gz-cmake#334 to support the cmake installation directory to be moved after the make install prefix, and continue to work without the need to set any special environment variable, as long as the library is compiled as shared. To avoid regressions and problems in Ubuntu Focal due to the use of std::filesystem, this new behaviour is only activated if the GZ_ENABLE_RELOCATABLE_INSTALL option is enabled, and its default value is OFF .

In particular, this PR defines a sdf::getSharePath() function that should be used in place of the SDF_SHARE_PATH macro to ensure that the library is relocatable.

Signed-off-by: Silvio Traversaro <silvio@traversaro.it>
Co-authored-by: Steve Peters <scpeters@openrobotics.org>
Co-authored-by: Ian Chen <ichen@openrobotics.org>

The windows builds were broken with -DBUILD_TESTING=OFF,
so move the target_link_libraries call outside the
`if (BUILD_TESTING)` logical block.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

Signed-off-by: Ian Chen <ichen@openrobotics.org>