This will simplify linking against the lib, both from the build-tree and
install-tree from other CMake projects.
CMakes find_package command uses these configs to locate the exported
targets for the library.
* Also changed so that all CMake options for the project are prepended
with JANSSON_ so that there is no ambiguity when including this as a
subdirectory in another CMake project.
- Also added better FindSphinx support so that we can detect the version and not have a fatal error if it's too old.
- Added support for building latex documentation with -DBUILD_LATEX. Off by default.
- Added suppor for building man pages with -DBUILD_MAN. On by default (unless sphins is too old to support it).
