Update devel/libbson to 1.3.0.

pkgsrc changes:
- Release tarballs cannot build man pages any more, use pre-built
  ones instead.

Changes since 1.2.1:
- Fix potential crash in bson_strncpy on Windows.
- Parse DBRefs correctly from JSON.
- CMake option to disable building tests: "cmake -DENABLE_TESTS:BOOL=OFF".
- Refactor the build system to declare library version in one place.
- Fix compiler warnings and errors, especially with Visual Studio 2015
  and IBM XL C.
- Combine environment's CFLAGS with configure options when building.

(fhajny)