Make short-circuit builds work

The build would fail because the target already exists. Use "-f" to
make it work. This is useful because glibc takes a while to build ;)

glibc.spec

@@ -1677,10 +1677,10 @@ cp benchtests/scripts/validate_benchout.py %{glibc_sysroot}%{_prefix}/libexec/gl
 # The #line directives gperf generates do not give the proper
 # file name relative to the build directory.
 pushd locale
-ln -s programs/*.gperf .
+ln -sf programs/*.gperf .
 popd
 pushd iconv
-ln -s ../locale/programs/charmap-kw.gperf .
+ln -sf ../locale/programs/charmap-kw.gperf .
 popd
 
 %if %{with docs}
@@ -1765,7 +1765,7 @@ for lib in lib lib64;  do
 		    exit 1
 		fi
 		set -x
-		ln -s $sltarget usr/$lib/$slbase
+		ln -sf $sltarget usr/$lib/$slbase
 	    done
 	fi
     done