fix: Use PACKAGE_PREFIX_DIR to ensure relocatable CMake install for conan

[Pro]

Using CMAKE_INSTALL_PREFIX causes build issues when you relocate the installed CMake package, i.e., as it is done in conan.

CMAKE_INSTALL_PREFIX should not be used in the cmake.in files directly. The @PACKAGE_INIT@ macro initializes PACKAGE_PREFIX_DIR correspondingly.

See also:
gazebosim/gz-cmake#129
gazebosim/sdformat#419
gazebosim/gazebo-classic#2782

cc @stefanbuettner

[caguero]

@Pro, thanks for the contribution! I tried out and it seems to work fine. Approving!

[traversaro]

Note that the change in the .pc file here could break Debian packaging for the reasons described in dartsim/dart#1527 (comment) . fyi @j-rivero

[caguero]

Thanks @traversaro! @j-rivero , do you foresee any issues?

[scpeters]

Note that the change in the .pc file here could break Debian packaging for the reasons described in dartsim/dart#1527 (comment) . fyi @j-rivero

I made that comment on dartsim, and yes I think it will break our .pc files on Ubuntu, since the relative path is not correct when installed to an architecture-dependent subfolder like /usr/lib/x86_64-linux-gnu/pkgconfig

[jslee02]

@scpeters Consider using the trick used in dartsim/dart#1529. It should work even for the case you described.