Fix CMake dependencies for Riegl RDB Plugin (#2219)

* Remove explicit include directory setting

The linked target `rdbcpp` already sets `target_include_directories`.
CMake propagates these directories, so no use adding them again.

* Link to PDAL provided Eigen vendor library

The previously used CMake variable `EIGEN_INCLUDE_DIRS` was empty
when no Eigen library was installed. Since PDAL base lib is built
with `vendor/eigen` in any case, using the very same Eigen library
should be the safest option for a PDAL plugin.

* Add include directory for JSONCPP library

When JSONCPP is not installed ie. the include files are in a common
directory that is searched for library includes anyway, explicitly
setting the include directory is required.

plugins/rdb/CMakeLists.txt

@@ -13,8 +13,8 @@ PDAL_ADD_PLUGIN(libname reader rdb
     ${PDAL_JSONCPP_LIB_NAME}
     rdbcpp)
 
-target_include_directories(${libname} PUBLIC ${RDB_INCLUDE_CPP_DIR})
-target_include_directories(${libname} PUBLIC ${EIGEN_INCLUDE_DIRS})
+target_include_directories(${libname} PUBLIC "${PDAL_VENDOR_DIR}/eigen")
+target_include_directories(${libname} PRIVATE ${PDAL_JSONCPP_INCLUDE_DIR})
 
 
 
@@ -29,7 +29,6 @@ if (BUILD_RDBLIB_TESTS)
     PDAL_ADD_TEST(${RDB_TEST_NAME}
         FILES test/RdbReaderTest.cpp
         LINK_WITH ${libname})
-    target_include_directories(${RDB_TEST_NAME} PUBLIC ${RDB_INCLUDE_CPP_DIR})
     target_include_directories(${RDB_TEST_NAME} PRIVATE
         ${PROJECT_BINARY_DIR}/plugins/rdb/test
         ${PROJECT_SOURCE_DIR}/plugins/rdb/io)