less /dev/shm/new-git/cinelerra/cinelerra-5.1/thirdparty/suil-0.10.6/build/
.lock-waf_linux2_build libsuil-0.a libsuil_x11_in_gtk2.so
.wafpickle-linux2-34018032-20 libsuil-0.so libsuil_x11_in_gtk3.so
c4che/ libsuil-0.so.0 src/
compile_commands.json libsuil-0.so.0.10.6 suil-0.pc
config.log libsuil_x11.so suil_config.h
look for config.log file there and at
less /dev/shm/new-git/cinelerra/cinelerra-5.1/thirdparty/lv2-1.18.0/build/config.log
both should detect gtk 2.18+ - yes.
Try to (temporarily) deinstall system-level lv2/suil packages before building cingg?