Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: vm/vm.a(cogit.o):/build/source/spur64src/vm/cointerp.h:358: multiple definition of `checkAllocFiller';
vm/vm.a(gcc3x-cointerp.o):/build/source/spur64src/vm/cointerp.h:358: first defined here