If WAYLAND_DEFINITIONS is set to "-I/usr/include/wayland", setting its
value as INTERFACE_COMPILE_DEFINITION leads to CMake emitting
"-D-I/usr/include/wayland" in the compiler flags. This breaks the
compilation of targets that call find_package(Wayland), such as libobs
and libobs-opengl, in gcc.
To avoid this, rename WAYLAND_DEFINITIONS to WAYLAND_COMPILE_FLAGS to
reflect that it contains ready-to-use compiler flags. Use
WAYLAND_COMPILE_FLAGS as INTERFACE_COMPILE_OPTIONS so it just gets
appended without adding the superfluous "-D".
