[CMake] Creating relocatable packages

[Zac Bergquist]

Hello,

I'm trying to fix a project so that its output is relocatable.  The project
is a library that itself depends on libpcap.  My goal is to be able to
import the library into my CMake application and have CMake automatically
link libpcap with it.

I'm using a toolchain file that invokes an ARM cross compiler, sets
CMAKE_INSTALL_PREFIX and appends it to CMAKE_FIND_ROOT_PATH.  This all
seems to work well.

What I'm seeing is that the exported files that get installed to
CMAKE_INSTALL_PREFIX contain absolute paths to libpcap on my machine, which
make them useless to other team members.

The first thing I changed was to use CONFIGURE_PACKAGE_CONFIG_FILE instead
of CONFIGURE_FILE.  This got rid of absolute paths in the generated CMake
config file.

However, I still have absolute paths in the CMake target import file that
gets generated.  The IMPORTED_LINK_INTERFACE_LIBRARIES target property has
an absolute path instead of one relative to CMAKE_INSTALL_PREFIX.

I've read the "Creating Relocatable Packages" [1] section of the CMake
packages documentation. I'm pretty sure this is what I'm running into, but
I don't quite understand how to solve it.  The documentation says that I
should be using imported targets, but I'm not sure how to do so.  The
library is just using its own FindPCAP.cmake file to locate libpcap.

Does anyone know of an example of a CMake package that imports other
libraries?  I could also try to create a minimal example project with this
setup if it would be helpful.

Thanks,
Zac

[1]:
https://cmake.org/cmake/help/v3.3/manual/cmake-packages.7.html#creating-relocatable-packages
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