src/modules/meson: make Opus custom modes optional for NetJack2
At least Gentoo makes the /usr/include/opus/opus_custom.h header optional, so just in case there's more such distros, the Opus codec support in NetJack2 will now get disabled, if it's not available. Not ideal from distro perspective but better than a build failure.
Edited by Niklāvs Koļesņikovs