SDL2: don't mix space-separated / lists in cmake config

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.