Nokogiri Compilation Error - Can't find libraries/headers

Trying to install a gem, but it can't find the headers, despite specifying them:

sudo gem install nokogiri -- --with-xml2-lib=/usr/local/lib --with-xml2-include=/usr/local/include/libxml2  --with-xml2-include=/usr/local/include/libxml2  --with-xslt-include=/usr/local/include/libxslt
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
    ERROR: Failed to build gem native extension.

/usr/bin/ruby extconf.rb --with-xml2-lib=/usr/local/lib --with-xml2-include=/usr/local/include/libxml2 --with-xml2-include=/usr/local开发者_JAVA技巧/include/libxml2 --with-xslt-include=/usr/local/include/libxslt
checking for #include <libxml/parser.h>
... no
-----
libxml2 is missing.  please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing dependencies.

The /usr/local/include/libxml2/libxml/parser.h file does exist, so I don't see why this isn't working.

EDIT: I'm on Centos 5.4

If you dig into /usr/lib/ruby/1.8/mkmf.rb you can figure out how to simulate the check that's happening during install. In my case I could do this by creating a file called conftest.c containing:

#include <libxml/parser.h>

Then try running the command that gets passed into the try_do function. In my case this was:

gcc -E -I. -I/usr/lib64/ruby/1.8/x86_64-linux -I. -I-I-I/opt/local/include -I-I-I/usr/local/include -I-I-I/usr/include -I-I-I/usr/include -I-I-I/usr/include/libxml2 -I/opt/local/include/libxml2 -I/usr/local/include/libxml2 -I-I/opt/local/include -I-I/usr/local/include -I-I/usr/include -I-I/usr/include -I-I/usr/include/libxml2 -I/opt/local/include -I/usr/local/include -I/usr/include -I/usr/include -I/usr/include/libxml2   -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -fno-strict-aliasing  -fPIC  -g -DXP_UNIX -O3 -Wall -Wcast-qual -Wwrite-strings -Wconversion -Wmissing-noreturn -Winline  conftest.c -o conftest.i

In my case I was missing gcc, which is probably not what you're hitting since you said you compiled from source. But hopefully if you run this command it should give you an idea of why you can't load in libxml2.

If you're interested in pre-built apparently EPEL has a rubygems-nokogiri package. Good chance I'll go this route in the end.