#1523 Build fails with clang-11: duplicate symbol: cfbWindowPrivateIndex, etc

[Yuri]

clang recently change the default from -fcommon to -fno-common and now the build fails:
Log: http://beefy18.nyi.freebsd.org/data/head-amd64-default/p550759_s366303/logs/errors/tightvnc-1.3.10_6.log (IPv6 URL)

ld: error: duplicate symbol: cfbGCPrivateIndex
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(cfbGCPrivateIndex) in archive cfb/libcfb.a
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(.bss+0x4) in archive cfb16/libcfb.a

ld: error: duplicate symbol: cfbWindowPrivateIndex
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(cfbWindowPrivateIndex) in archive cfb/libcfb.a
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(.bss+0x0) in archive cfb16/libcfb.a

ld: error: duplicate symbol: cfbPuntCopyPlane
>>> defined at cfbbitblt.c
>>>            cfbbitblt.o:(cfbPuntCopyPlane) in archive cfb/libcfb.a
>>> defined at cfbbitblt.c
>>>            cfbbitblt.o:(.bss+0x0) in archive cfb16/libcfb.a

ld: error: duplicate symbol: cfbGCPrivateIndex
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(cfbGCPrivateIndex) in archive cfb/libcfb.a
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(.bss+0x4) in archive cfb32/libcfb.a

ld: error: duplicate symbol: cfbWindowPrivateIndex
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(cfbWindowPrivateIndex) in archive cfb/libcfb.a
>>> defined at cfballpriv.c
>>>            cfballpriv.o:(.bss+0x0) in archive cfb32/libcfb.a

ld: error: duplicate symbol: cfbPuntCopyPlane
>>> defined at cfbbitblt.c
>>>            cfbbitblt.o:(cfbPuntCopyPlane) in archive cfb/libcfb.a
>>> defined at cfbbitblt.c
>>>            cfbbitblt.o:(.bss+0x0) in archive cfb32/libcfb.a

Please do not define symbols in the headers.