`NO_DEFAULT_PATH` was added in 565eb6b6fd3e6b2c9275490c3c07e585b0942a92.
However this breaks Nixpkgs/NixOS, which installs everything to separate
prefixes and adds compiler flags to supply them.
This commit removes `NO_DEFAULT_PATH`. Instead, it looks for a header
file specific to SDL 2.0 and not in SDL 1.2.
Both of SDL 1.2 and 2.0 includes `SDL.h`, so `FIND_PATH` sometimes gets
confused when both of them are installed. This commit fixes that by
limiting the search paths to directories specific to SDL 2.0.
