Error building gems in Mountain Lion - ruby

Question

I just bought a new MacBook Pro (obviously w/ML). First thing I'm doing is installing development tools. I installed Ruby with rvm and I've also got Homebrew installed. I had to download the XCode command line tools.
$ gcc -v
Using built-in specs.
Target: i686-apple-darwin11
Configured with: /private/var/tmp/llvmgcc42/llvmgcc42-2336.11~67/src/configure --disable-checking --enable-werror --prefix=/Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2 --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-prefix=llvm- --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib --build=i686-apple-darwin11 --enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2336.11~67/dst-llvmCore/Developer/usr/local --program-prefix=i686-apple-darwin11- --host=x86_64-apple-darwin11 --target=i686-apple-darwin11 --with-gxx-include-dir=/usr/include/c++/4.2.1
Thread model: posix
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
I was able to install the mongo gem fine, but when I try to install nokogiri, I get the following errors:
$ gem install nokogiri
Building native extensions. This could take a while...
ERROR: Error installing nokogiri:
ERROR: Failed to build gem native extension.
/Users/johnsmith/.rvm/rubies/ruby-1.9.3-p327/bin/ruby extconf.rb
checking for libxml/parser.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
After that message, it lists a whole bunch of configuration options, and then says:
/Users/johnsmith/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/mkmf.rb:369:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
It seems that I have the development tools, so I can't figure out the problem. Any thoughts?

Answer 1

I had to look around a while, but I found the solution at github:
https://github.com/sparklemotion/nokogiri/issues/442#issuecomment-7978408
libxml2 is missing.
I found one extra step was required to solve the above for Homebrew
0.9 on Mac OS X 10.8 Mountain Lion and that is to create a "/usr/bin/gcc-4.2" link:
brew install libxml2 libxslt
brew link libxml2 libxslt
wget http://ftp.gnu.org/pub/gnu/libiconv/libiconv-1.13.1.tar.gz
tar xvfz libiconv-1.13.1.tar.gz
cd libiconv-1.13.1
./configure --prefix=/usr/local/Cellar/libiconv/1.13.1
make
sudo make install
sudo ln -s /usr/bin/gcc /usr/bin/gcc-4.2
gem install nokogiri -- --with-xml2-include=/usr/local/Cellar/libxml2/2.8.0/include/libxml2 --with-xml2-lib=/usr/local/Cellar/libxml2/2.8.0/lib --with-xslt-dir=/usr/local/Cellar/libxslt/1.1.26 --with-iconv-include=/usr/local/Cellar/libiconv/1.13.1/include --with-iconv-lib=/usr/local/Cellar/libiconv/1.13.1/lib

Answer 2

In this case, you need the development headers for libxml which are not provided as part of the compiler tools. Usually you will need to install one of Homebrew or MacPorts to help install these dependencies.
The hint here is somewhat hard to spot in the noise if you're not familiar, but is:
checking for libxml/parser.h... *** extconf.rb failed ***
The .h files are provided with the development packages and are necessary to compile extensions against those libraries.