Meson builds libigt as a shared library, and executables naturally
have to find it at runtime. Using default options puts the library to
a normal search paths, but any modifications to the directory options
or a non-conventional prefix setting makes using LD_LIBRARY_PATH or
other library search means mandatory.

Add a build option 'use_rpath' (default: false) that makes meson set
up DT_RUNPATH at install time, pointing to the library with a path
relative to the executable, using $ORIGIN. That way the installed
executables find the library even when not installed to exactly the
build-time configured prefix path, a setup CI occasionally uses.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Tested-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>