取り敢えず、NetBSD currentでの状況です。pkgsrc的なことは特にせず、ある
がままの環境でのコンパイルを試みました。

% uname -sr
NetBSD 1.6ZE
% sh configure --enable-shared
...
gcc -shared  -L"/var/tmp/ruby-1.8.1" -Wl,-R"/var/tmp/ruby-1.8.1" -L"/usr/local/lib" -Wl,-R"/usr/local/lib" -o strscan.so strscan.o  -Wl,-R -Wl,/usr/local/lib -L/usr/local/lib -L. -lruby  -lcrypt -lm  -lc

ここでカレント・ディレクトリの絶対パスを、ld(1)の-Rに指定してしまって
いるのが、気になります。また、この後以下のようなエラーになりました。

compiling syck
gcc -fPIC -g -O2 -fPIC -pipe -I. -I/var/tmp/ruby-1.8.1 -I/var/tmp/ruby-1.8.1 -I/var/tmp/ruby-1.8.1/ext/syck -DHAVE_ST_H -c bytecode.c
...
gcc -fPIC -g -O2 -fPIC -pipe -I. -I/var/tmp/ruby-1.8.1 -I/var/tmp/ruby-1.8.1 -I/var/tmp/ruby-1.8.1/ext/syck -DHAVE_ST_H -c yaml2byte.c
yaml2byte.c:15:22: yamlbyte.h: No such file or directory
yaml2byte.c: In function `bytestring_alloc':
yaml2byte.c:42: warning: integer constant is too large for "long" type
yaml2byte.c: In function `bytestring_append':
yaml2byte.c:56: warning: integer constant is too large for "long" type
yaml2byte.c: In function `bytestring_extend':
yaml2byte.c:89: warning: integer constant is too large for "long" type
yaml2byte.c:90: warning: integer constant is too large for "long" type
yaml2byte.c:92: error: `YAMLBYTE_ANCHOR' undeclared (first use in this function)
yaml2byte.c:92: error: (Each undeclared identifier is reported only once
yaml2byte.c:92: error: for each function it appears in.)
yaml2byte.c:96: error: `YAMLBYTE_ALIAS' undeclared (first use in this function)
yaml2byte.c: In function `syck_yaml2byte_handler':
yaml2byte.c:134: error: `YAMLBYTE_ANCHOR' undeclared (first use in this function)
yaml2byte.c:139: error: `YAMLBYTE_TRANSFER' undeclared (first use in this function)
yaml2byte.c:153: error: `YAMLBYTE_SCALAR' undeclared (first use in this function)
yaml2byte.c:163: error: `YAMLBYTE_CONTINUE' undeclared (first use in this function)
yaml2byte.c:172: error: `YAMLBYTE_NEWLINE' undeclared (first use in this function)
yaml2byte.c:176: error: `YAMLBYTE_NULLCHAR' undeclared (first use in this function)
yaml2byte.c:187: error: `YAMLBYTE_SEQUENCE' undeclared (first use in this function)
yaml2byte.c:194: error: `YAMLBYTE_END_BRANCH' undeclared (first use in this function)
yaml2byte.c:197: error: `YAMLBYTE_MAPPING' undeclared (first use in this function)
*** Error code 1

Stop.
make: stopped in /var/tmp/ruby-1.8.1/ext/syck
*** Error code 1

Stop.
make: stopped in /var/tmp/ruby-1.8.1

-- 
神戸 隆博 / Takahiro Kambe