When configuring Ruby-1.6.1 on solaris 2.5.1 I got:
...
checking count field in FILE structures... _cnt
checking whether ELF binaries are produced... yes
checking whether OS depend dynamic link works... yes
./configure: : cannot execute
./configure: ${prefix}/lib/ruby/site_ruby: not found
creating config.h
updating cache ./config.cache
creating ./config.status
creating Makefile
creating ext/extmk.rb
brains hgs 61 %> 

sh -vx ./configure yields:

...
case "$target_os" in
  cygwin*|mingw*|*djgpp*)
    RUBY_LIB_PREFIX="/lib/ruby"
    ;;
  *)
    RUBY_LIB_PREFIX="${prefix}/lib/ruby"
    ;;
esac
RUBY_LIB_PREFIX=/usr/local/lib/ruby
RUBY_LIB_PATH="${RUBY_LIB_PREFIX}/${MAJOR}.${MINOR}"
RUBY_LIB_PATH=/usr/local/lib/ruby/1.6

# Check whether --with-sitedir or --without-sitedir was given.
if test "${with_sitedir+set}" = set; then
  withval="$with_sitedir"
  sitedir=$withval
else
  sitedir='${prefix}/lib/ruby/site_ruby'
fi
+ test  = set 
sitedir=${prefix}/lib/ruby/site_ruby

SITE_DIR="`eval "echo ${sitedir}"`"
+  
./configure: : cannot execute
+ ${prefix}/lib/ruby/site_ruby 
+ eval echo 
+ echo 
SITE_DIR=
./configure: ${prefix}/lib/ruby/site_ruby: not found
case "$target_os" in
  cygwin*|mingw*|*djgpp*)
    RUBY_SITE_LIB_PATH="`expr "$SITE_DIR" : "$prefix\(/.*\)"`" ||
    RUBY_SITE_LIB_PATH="$SITE_DIR";;
  *)
    RUBY_SITE_LIB_PATH="$SITE_DIR";;
esac
RUBY_SITE_LIB_PATH=
RUBY_SITE_LIB_PATH2="${RUBY_SITE_LIB_PATH}/${MAJOR}.${MINOR}"
RUBY_SITE_LIB_PATH2=/1.6

...

if that is any help.
	Thank you,
	Hugh
	hgs / dmu.ac.uk