Something

cd ruby && /usr/local/bin/ruby extconf.rb && gmake EPREFIX=/usr/local sitedir=/usr/local/lib/ruby
checking for rrd_create() in -lrrd... yes
creating Makefile
gmake[4]: Entering directory `/usr/ports/net/rrdtool/work/rrdtool-1.2.23/bindings/ruby'
Makefile:144: Commands were specified for file `RRD.so' at Makefile:114,
Makefile:144: but `RRD.so' is now considered the same file as `/usr/local/lib/ruby/1.8/i386-freebsd6/RRD.so'.
Makefile:144: Commands for `/usr/local/lib/ruby/1.8/i386-freebsd6/RRD.so' will be ignored in favor of those for `RRD.so'.
gmake[4]: Circular /usr/local/lib/ruby/1.8/i386-freebsd6/RRD.so <- /usr/local/lib/ruby/1.8/i386-freebsd6/RRD.so dependency dropped.
cc -I. -I. -I/usr/local/lib/ruby/1.8/i386-freebsd6 -I. -I../../src  -fPIC -O2 -fno-strict-aliasing -pipe    -fPIC  -c main.c
/usr/bin/install -c -o root -g wheel -m 0755 RRD.so /usr/local/lib/ruby/1.8/i386-freebsd6
install: RRD.so: No such file or directory
gmake[4]: *** [/usr/local/lib/ruby/1.8/i386-freebsd6/RRD.so] Error 71
gmake[4]: Leaving directory `/usr/ports/net/rrdtool/work/rrdtool-1.2.23/bindings/ruby'
gmake[3]: *** [ruby] Error 2
gmake[3]: Leaving directory `/usr/ports/net/rrdtool/work/rrdtool-1.2.23/bindings'
gmake[2]: *** [all-recursive] Error 1
gmake[2]: Leaving directory `/usr/ports/net/rrdtool/work/rrdtool-1.2.23/bindings'
gmake[1]: *** [all-recursive] Error 1
gmake[1]: Leaving directory `/usr/ports/net/rrdtool/work/rrdtool-1.2.23'
gmake: *** [all] Error 2
*** Error code 2

Stop in /usr/ports/net/rrdtool.
*** Error code 1

Stop in /usr/ports/net/rrdtool.