Exuberant ctags generates following tags file

	$ ctags -o - --C-kinds='x'  input.h
	C	/home/jet/var/ctags/Units/review-needed.r/extern_variable.h.t/input.h	/^class C;$/;"	x
	S	/home/jet/var/ctags/Units/review-needed.r/extern_variable.h.t/input.h	/^struct S;$/;"	x
	a	/home/jet/var/ctags/Units/review-needed.r/extern_variable.h.t/input.h	/^extern int a;$/;"	x
	b	/home/jet/var/ctags/Units/review-needed.r/extern_variable.h.t/input.h	/^extern struct B b;$/;"	x

Universal ctags generates following tags file

	$ ./ctags -o - --C-kinds='x'  input.h
	a	/home/jet/var/ctags/Units/review-needed.r/extern_variable.h.t/input.h	/^extern int a;$/;"	x	typeref:typename:int
	b	/home/jet/var/ctags/Units/review-needed.r/extern_variable.h.t/input.h	/^extern struct B b;$/;"	x	typeref:struct:B

I marked this .b. We have to consider which behavior is better.
