fails to build with "multiple definition" errors

[umlaeute]

building xjadeo fails wtih a linker error:

gcc -Wall -g -O3 -pthread -I/usr/include/x86_64-linux-gnu   -I/usr/include/freetype2 -I/usr/include/libpng16    -D_GNU_SOURCE=1 -D_REENTRANT -I/usr/include/SDL    "-DSUBVERSION=\"\"" -g -O2 -fdebug-prefix-map=/builds/multimedia-team/xjadeo/debian/output/xjadeo-0.8.9=. -fstack-protector-strong -Wformat -Werror=format-security  -Wl,-z,relro -Wl,-z,now -Wl,--as-needed -o xjadeo xjadeo-main.o xjadeo-xjadeo.o xjadeo-remote.o xjadeo-mqueue.o xjadeo-xjosc.o xjadeo-configfile.o xjadeo-common.o xjadeo-common_jack.o xjadeo-jack.o xjadeo-ltc-jack.o xjadeo-midi.o xjadeo-freetype.o xjadeo-smpte.o xjadeo-display.o xjadeo-display_x_dnd.o xjadeo-display_x_dialog.o xjadeo-libsofd.o xjadeo-display_mac.o xjadeo-display_x11.o xjadeo-display_sdl.o xjadeo-weak_libjack.o xjadeo-gtime.o xjadeo-display_glx.o    -lrt -lavformat -lavcodec -lavutil -lswscale -lXext -lXv -lportmidi -lporttime -lasound -lfreetype -lImlib2 -lXpm -lX11 -llo -lpthread -lSDL -lltc -lm -lGLU -lGL -lm -ljack -lpthread     -L/usr/X11R6/lib
/usr/bin/ld: xjadeo-configfile.o:./src/xjadeo/configfile.c:93: multiple definition of `start_ontop'; xjadeo-main.o:./src/xjadeo/main.c:173: first defined here
/usr/bin/ld: xjadeo-configfile.o:./src/xjadeo/configfile.c:94: multiple definition of `start_fullscreen'; xjadeo-main.o:./src/xjadeo/main.c:174: first defined here

full build logs can be found here:
https://salsa.debian.org/multimedia-team/xjadeo/-/jobs/943667#L1981

i guess this is because in src/xjadeo/config.h the two variables are not declared extern.

[umlaeute]

btw, that's with gcc10.
afaik, there was no problem with gcc9.