Fixes#103346. The environment var for finding the cmake companion libs
headers was missing from the SDL2::SDL2 target, it only worked when
using SDL2_INCLUDE_DIRS.
The SDL2_PATH environment variable is space-separated. However, CMake
lists are semicolon separated, and doing set(list "a" ${var}) will end
up with list containing "a;b c d". This causes downstream users to fail
parsing the include dirs list.
Normalize the include dirs list to a normal CMake semicolon-separated
list using separate_arguments.