Add the RPATH within the CMake setup.

[fruffy]

Not entirely sure whether this is the right way to do it.

Fixes #51.

[ChrisDodd]

You might need to escape the $ somehow so it doesn't get expanded as a make or shell variable. In a Makefile, you generally need '$$ORIGIN' for that. Not sure what cmake needs.

[fruffy]

Allegedly, CMake is okay with $ORIGIN as is, only ${} needs to be escaped. But unfortunately I am unable to test that right now.

[vgurevich]

Thanks, @fruffy -- works for me:

ubuntu@ip-172-31-32-12:~/open-p4studio$ echo $LD_LIBRARY_PATH    # It is not set

ubuntu@ip-172-31-32-12:~/open-p4studio$ ldd $SDE_INSTALL/bin/p4c-barefoot
	linux-vdso.so.1 (0x00007ffe1434b000)
	libboost_iostreams.so.1.67.0 => /home/ubuntu/open-p4studio/install/bin/../lib/libboost_iostreams.so.1.67.0 (0x00007fc254daf000)
	libprotobuf.so.25.0.0 => /home/ubuntu/open-p4studio/install/bin/../lib/libprotobuf.so.25.0.0 (0x00007fc254562000)
	libabsl_log_internal_check_op.so.2308.0.0 => /home/ubuntu/open-p4studio/install/bin/../lib/libabsl_log_internal_check_op.so.2308.0.0 (0x00007fc254558000)
	libabsl_status.so.2308.0.0 => /home/ubuntu/open-p4studio/install/bin/../lib/libabsl_status.so.2308.0.0 (0x00007fc25452e000)
	libabsl_log_internal_message.so.2308.0.0 => /home/ubuntu/open-p4studio/install/bin/../lib/libabsl_log_internal_message.so.2308.0.0 (0x00007fc25450e000)
	libabsl_log_internal_nullguard.so.2308.0.0 => /home/ubuntu/open-p4studio/install/bin/../lib/libabsl_log_internal_nullguard.so.2308.0.0 (0x00007fc254507000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fc2544e8000)
...