Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: .libs/hash_drv.o:/build/dspam-3.10.2/src/util.h:96: multiple definition of `verified_user';
.libs/libdspam.o:/build/dspam-3.10.2/src/util.h:96: first defined here