Set install RPATH in TKernel

GNU ld has a new-dtags feature which inserts DT_RUNPATH section instead
of DT_RPATH section in ELF shared libraries. Unlike DT_RPATH, DT_RUNPATH
is not transitive.
Draw plugins are loaded by OSD_SharedLibrary::DlOpen, this is why library
paths must be hardcoded into TKernel.

As TKernel does not depend on other libraries, no RPATH section is
automatically inserted by CMake.  To workaround this, INSTALL_RPATH
and BUILD_WITH_INSTALL_RPATH properties of TKernel are explicitly set.

The new OCE_TKERNEL_BUILD_WITH_INSTALL_RPATH option is provided; it is
ON by default, and can be disabled if this change causes trouble.

Fix #483.
Merge pull request #495 from tpaviot/review/set-TKernel-RPATH

Set install RPATH in TKernel