srt recipe does not have the correct install_name on macOS

Submitted by Nirbheek Chauhan

Link to original bug (#796616)

Description

The install_name is not set as the absolute path to the library, due to which the srt plugin fails to load because it can't find libsrt.1.dylib.

This is likely because both these use cmake and don't use the correct incantation to set the install name as the absolute path. Openjpeg also uses cmake and does this correctly, so we should just copy what it does.