fixes

[aabacchus]

Some utilities require -fcommon to compile on my system.

[sewnie]

solves the following for me as well:

/usr/bin/ld: zopen.o:(.bss+0x0): multiple definition of `pmode'; main.o:(.bss+0x24): first defined here
/usr/bin/ld: gzopen.o:(.bss+0x0): multiple definition of `pmode'; main.o:(.bss+0x24): first defined here
/usr/bin/ld: nullopen.o:(.bss+0x4): multiple definition of `pmode'; main.o:(.bss+0x24): first defined here