Add pkg-config support

[jplatte]

Resolves #18

[cdunn2001]

Thanks!

[jplatte]

No problem :)

I now realized that it's not complete though. I tested the generation but not the usage and the pkg-config file is missing a "Version" definition. If I knew how to get the version number in CMake, I could easily create another small patch, but I don't find anything concerning versioning in CMakeLists.txt.

[cdunn2001]

If you think it's worthwhile, you can recover part of the CMake rules for the version-string from a few commits back, where I deleted it. I certainly don't mind a version string in header files or pkg-config, but it must not confuse the revision-control system. The problem was that people were running amalgamate.py before running cmake, so they lacked the generated version.h file. But if only cmake generates the pkg-config, then it's fine.

[jplatte]

For the .pc file, the version string is mandatory (I accidentaly used another incomplete file as template), so it would be needed for pkg-config support. I don't know what amalgamate.py is though. It's up to you to decide what to do...

On a related note: When testing the installation of jsoncpp, I saw a jsoncpp.so.SOVERSION being installed, which obviously seems wrong. Should I create a seperate issue for this or do you know what this is about and you're able to fix it when recovering the version information in CMake?

[cdunn2001]

SOVERSION might be related to the SONAME problem in issue #43.

[cdunn2001]

JSONCPP_VERSION and JSONCPP_VERSION_MAJOR are now available. (See #44.) I don't know which you want. Feel free to update your pkg-config template.