Fix generation of pkg-config file with absolute includedir/libdir.

[lopsided98]

The CMake variables CMAKE_INSTALL_LIBDIR and CMAKE_INSTALL_INCLUDEDIR are not guaranteed to be relative paths. If they are absolute, an invalid pkg-config file is generated.

See https://github.com/jtojnar/cmake-snips#concatenating-paths-when-building-pkg-config-files for more information.

[coveralls]

Coverage remained the same at 93.801% when pulling 789f29e on lopsided98:pkgconfig-paths into bf0cfa5 on open-source-parsers:master.

[dota17]

LGTM.

[dota17]

Can this line move down a bit? I think there should be a better place.

[lopsided98]

I moved it down. If you have somewhere specific you want it let me know.

[dota17]

Thanks.

[ferdnyc]

Related to this PR (but also just jsoncpp installation in general), note that work is underway to (hopefully) come up with a design for built-in pkg-config features for CMake's install(EXPORT ...) mode. (The notion is that EXPORT would optionally generate .pc files to complement CMake's native EXPORTED configurations, so that CMake-built and -installed dependencies are visible to build systems other than CMake.)

One major hurdle to overcome in the design so far has been the question of input dependency handling. Particularly in a presumptive future where CMake, when generating the build system for some project, may have to contend with dependencies that are defined by a mix of native CMake EXPORTED target configurations, .pc files that CMake also generated, and .pc files that came from another build system. And some of those may be redundant with each other.

As a CMake project which also generates and installs .pc files, jsoncpp is very much the target audience for the (proposed) new features. I'd encourage anyone interested to look over the design-in-progress and offer any input you may have, so that the final implementation hopefully will meet or exceed both jsoncpp's current and expected future requirements.