CMakeLists.txt: find build dependency liborc-0.4

[davidbullado]

Hi :)

I encountered this error when building the project: No rule to make target '/usr/lib/x86_64-linux-gnu/liborc-0.4.so'. This is because liborc was not installed on my Ubuntu 20.04 distribution. Therefore, I thought it would be helpful to have a more user-friendly warning before running the make command. That's why I added this check.

I hope it will be useful for other users :)

[bastibl]

This module does not use or link against liborc itself. I assume you have left-over artifacts from a GNU Radio installation that uses liborc and meanwhile uninstalled the library.