Modernize CMake build system

cmake/FixInterfaceIncludes.cmake

@@ -0,0 +1,24 @@
+# The entire codebase quire reasonably does things like #include <SDL2/SDL.h> or <epoxy/gl.h>
+# CMake apparently doesn't think you should be doing this, so just includes $PREFIX/include/packagename for a given
+# package as include directories when using `target_link_libraries` with an imported target, this hacky function fixes
+# that up so includes can keep working as they always did but we can still use fancy imported targets.
+# This is stupid.
+
+function(fix_interface_includes)
+    foreach (target ${ARGN})
+        set(NEW_DIRS)
+        get_target_property(DIRS "${target}" INTERFACE_INCLUDE_DIRECTORIES)
+
+        foreach (DIR ${DIRS})
+            get_filename_component(PARENT_DIR "${DIR}" DIRECTORY)
+
+            if (PARENT_DIR MATCHES "include$")
+                list(APPEND NEW_DIRS "${PARENT_DIR}")
+            endif()
+        endforeach()
+
+        list(APPEND DIRS ${NEW_DIRS})
+        set_target_properties("${target}" PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${DIRS}")
+    endforeach()
+endfunction()
+