Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: ../../src/lib/libactive_directory.a(active_directory.o):/build/adtool-1.3.3/src/lib/active_directory.h:31:
multiple definition of `system_config_file'; adtool.o:/build/adtool-1.3.3/src/tools/../../src/lib/active_directory.h:31: first defined here