boards/posix: Handle SDL2 compile vars with multiple values

When transforming SDL2_LIBRARIES and SDL2_INCLUDE_DIRS into compile
options, each member needs to be preceeded by -l or -I. Use the cmake
list(TRANFORM) operation to prepend the flag to each list element instead
of just prepending the flag to the entire list.

Signed-off-by: Keith Packard <keithp@keithp.com>