Ruby – gem install mongrel fails with ruby 1.9.1

I initiated myself into rails development yesterday.
I installed ruby 1.9.1, rubygems and rails.
Running gem install mongrel worked fine and ostensibly installed mongrel too. I am slightly puzzled because:

• script/server starts webrick by default
• which mongrel returns nothing
• locate mongrel returns lots of entries like

/Developer/SDKs/MacOSX10.5.sdk/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/mongrel-1.0.1
/Developer/SDKs/MacOSX10.5.sdk/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/mongrel-1.0.1/lib
/Developer/SDKs/MacOSX10.5.sdk/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/gems/1.8/gems/mongrel-1.0.1/lib/mongrel
.
.
.
/usr/local/bin/mongrel_rails
/usr/local/lib/ruby/gems/1.9.1/cache/mongrel-1.1.5.gem
/usr/local/lib/ruby/gems/1.9.1/doc/actionpack-2.3.2/rdoc/files/lib/action_controller/vendor/rack-1_0/rack/handler/evented_mongrel_rb.html
/usr/local/lib/ruby/gems/1.9.1/doc/actionpack-2.3.2/rdoc/files/lib/action_controller/vendor/rack-1_0/rack/handler/mongrel_rb.html
/usr/local/lib/ruby/gems/1.9.1/doc/actionpack-2.3.2/rdoc/files/lib/action_controller/vendor/rack-1_0/rack/handler/swiftiplied_mongrel_rb.html
/usr/local/lib/ruby/gems/1.9.1/gems/actionpack-2.3.2/lib/action_controller/vendor/rack-1.0/rack/handler/evented_mongrel.rb
/usr/local/lib/ruby/gems/1.9.1/gems/actionpack-2.3.2/lib/action_controller/vendor/rack-1.0/rack/handler/mongrel.rb
/usr/local/lib/ruby/gems/1.9.1/gems/actionpack-2.3.2/lib/action_controller/vendor/rack-1.0/rack/handler/swiftiplied_mongrel.rb
/usr/local/lib/ruby/gems/1.9.1/gems/mongrel-1.1.5
.
.
.

Does look like I have mongrel installed (both the default installation and my custom install). So why doesn't which mongrel return something.

Also trying to reinstall mongrel using gem install mongrel returns throws its own set of exceptions:

Building native extensions.  This could take a while...
ERROR:  Error installing mongrel:
    ERROR: Failed to build gem native extension.

/usr/local/bin/ruby extconf.rb install mongrel
checking for main() in -lc... yes
creating Makefile

make
gcc -I. -I/usr/local/include/ruby-1.9.1/i386-darwin9.7.0 -I/usr/local/include/ruby-1.9.1/ruby/backward -I/usr/local/include/ruby-1.9.1 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -fno-common -D_XOPEN_SOURCE=1 -O2 -g -Wall -Wno-parentheses  -fno-common -pipe -fno-common  -o http11.o -c http11.c
http11.c: In function 'http_field':
http11.c:77: error: 'struct RString' has no member named 'ptr'
http11.c:77: error: 'struct RString' has no member named 'len'
http11.c:77: warning: left-hand operand of comma expression has no effect
http11.c:77: warning: statement with no effect
http11.c: In function 'header_done':
http11.c:172: error: 'struct RString' has no member named 'ptr'
http11.c:174: error: 'struct RString' has no member named 'ptr'
http11.c:176: error: 'struct RString' has no member named 'ptr'
http11.c:177: error: 'struct RString' has no member named 'len'
http11.c: In function 'HttpParser_execute':
http11.c:298: error: 'struct RString' has no member named 'ptr'
http11.c:299: error: 'struct RString' has no member named 'len'
make: *** [http11.o] Error 1