Use relative path to includedirs.

[alex-rass-88]

Full path for target_include_directories generates ugly paths on windows

This fix generates relative include paths for CMake.

Before:

target_include_directories("lua" PRIVATE
  /C/dev/projects/tests/third/lua/include
  /C/dev/projects/tests/third/lua/src
)

After:

target_include_directories("lua" PRIVATE
  ${CMAKE_CURRENT_SOURCE_DIR}/../../third/lua/include
  ${CMAKE_CURRENT_SOURCE_DIR}/../../third/lua/src
)

[Jarod42]

I think it might be problematic for paths which are not relative (d:/)
Is ${CMAKE_CURRENT_SOURCE_DIR}/ needed BTW, relative path use already that, no?

[alex-rass-88]

Cmake sometimes misinterprets relative paths. Therefore, it is advisable to indicate where to start.
For complete paths, there are externalincludedirs