The option -fcommon controls whether uninitialized global
variables are placed in the COMMON section. If the option is given
in the negated form, -fno-common, variables are not placed in the
COMMON section. They are placed in the same sections as gcc does.

If the variables are not placed in the COMMON section merging of
tentative definitions is inhibited and multiple definitions lead
to a linker error, as it does for gcc.