CMake target_include_directories

I would like to use CMake's FetchContent with Xerces.
Unfortunately, since Xerces does not specify its include directories, this is not possible.

The fix is really easy and short:

Update src/CMakeLists.txt with the following code:

target_include_directories(
  xerces-c
  PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
)