common: autotools: add linking to xdot

common: autotools: add linking to xdot

E.g. lib/common/emit.c. uses parseXDotF from lib/xdot/xdot.c.

This removes the need to link to xdot for downstream libraries that do
not explicitly use xdot.

Fixes these errors using autotools with MinGW:

C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: ../../lib/common/.libs/libcommon_C.a(input.o): in function `graph_cleanup':
C:\Users\magja\graphviz\lib\common/input.c:881: undefined reference to `__imp_freeXDot'
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: ../../lib/common/.libs/libcommon_C.a(emit.o): in function `init_xdot':
C:\Users\magja\graphviz\lib\common/emit.c:58: undefined reference to `__imp_parseXDotF'