build: cmake: fix target_include_directories

[fcelda]

The target_include_directories option doesn't work as expected. I noticed that when trying to include the libmaxminddb as a CMake subdirectory into a different project.

Here is a simple reproducer:

project(reproducer)
add_subdirectory("libmaxminddb")

CMake will fail on building that project with the following error:

CMake Error in libmaxminddb/CMakeLists.txt:
  Target "maxminddb" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "/tmp/repro/libmaxminddb/"

  which is prefixed in the build directory.

The problem seems to be two-fold:

• The variable in $<BUILD_INTERFACE:${VAR}> must be a single path.
• The paths provided for build interface must not be relative.

The attached patch fixes the problem.

[oschwald]

👍