Add explicit linking against libz and libm

Add explicit linking against libz and libm

This fixes the following link errors on Fedora 13+

/usr/bin/ld: libviking.a(vikviewport.o): undefined reference to symbol 'tan@@GLIBC_2.2.5'
/usr/bin/ld: note: 'tan@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line

/usr/bin/ld: libviking.a(dem.o): undefined reference to symbol 'inflateInit2_'
/usr/bin/ld: note: 'inflateInit2_' is defined in DSO /lib64/libz.so.1 so try adding it to the linker command line

http://fedoraproject.org/w/index.php?title=UnderstandingDSOLinkChange