sdl: fix build of other sdl components

The post installation hook of SDL was not being executed, due to an
incorrect usage of the infrastructure. Moreover, it did not patch
prefix/exec_prefix in sdl-config as is needed. In addition to that, we
remove the -Wl,-rpath option from sdl-config, since it is not needed.

This fixes at least the build of sdl_image, and probably of other sdl
components as well.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>