Can you try:
CFLAGS="-march=native -O3" CXXFLAGS="-march=native -O3" ./configure
gcc (Gentoo 15.2.1_p20251122 p4) 15.2.1 20251122
Both systems tested are AMD Ryzen 5, and older Athlon X4.
Of course you are right that it errs out with O3 (O2 obviously works). Do you have a recommended fix? -- I can not code in C/C+ but I can test. I tried looking online for a code solution that I could understand and found nothing that made sense to me.
Just as an experiment and for fun I tried compiling --without-libzmpeg (either O2 or O3), it fails in cinelerra/Makefile with this error:
timebar.h:110:20: note: by ‘static int OutPointGUI::get_y(MWindow*, TimeBar*)’
110 | static int get_y(MWindow *mwindow, TimeBar *timebar);
| ^~~~~
make[2]: Leaving directory '/mnt0/cinelerra-5.1/cinelerra'
make[1]: *** [Makefile:600: all-recursive] Error 1
make[1]: Leaving directory '/mnt0/cinelerra-5.1'
make: *** [Makefile:547: all] Error 2