Enabled add_subdirectory() use using target_include_directories

[Dawars]

Hi, Great CMake file, it's easily readable! I modified it to enable using it as a library from code using add_subdirectory.

I replaced include_directories with target_include_directories.

Example use:

1. Add submodule/clone repo to libs folder
2. Add following to CMake file:

add_subdirectory(libs/magsac)

target_link_libraries(YourProgram PUBLIC MAGSAC GraphCutRANSAC)