Rename PKG_SOURCE. No point in matching with BUILD_DIR.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Just blindly copying the include files is not quite correct. The build
system merges all of them into one file.

A pkgconfig file also gets generated.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Add BUILDONLY since this is only used for the headers.

Rearranged variables for consistency between packages.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

JSON for Modern C++ library.
Design goals: intuitive syntax, trivial integration, serious testing.
https://nlohmann.github.io/json/
IPFS C++ HTTP API client library depends on this library.

Signed-off-by: Leonid Esman <leonid.esman@gmail.com>