pkg-config can't find libxml2.pc on OS X 10.8 Mountain Lion

[smackesey]

When running gem install gtksourceview2, I get the following error:

    *** 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.

Provided configuration options:
  --with-opt-dir
  --without-opt-dir
  --with-opt-include
  --without-opt-include=${opt-dir}/include
  --with-opt-lib
  --without-opt-lib=${opt-dir}/lib
  --with-make-prog
  --without-make-prog
  --srcdir=.
  --curdir
  --ruby=/Users/seanmackesey/.rbenv/versions/1.9.3-p385/bin/ruby
  --with-pkg-config
  --without-pkg-config
  --with-override-variables
  --without-override-variables
/Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:239:in `parse_pc': .pc for libxml-2.0 doesn't exist. (RuntimeError)
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:181:in `declaration'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:130:in `requires_private'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:311:in `all_required_packages'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:195:in `collect_cflags'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:134:in `cflags'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:196:in `block in collect_cflags'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:195:in `collect'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:195:in `collect_cflags'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:134:in `cflags'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:364:in `cflags'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/pkg-config-1.1.4/lib/pkg-config.rb:415:in `have_package'
  from /Users/seanmackesey/.rbenv/versions/1.9.3-p385/lib/ruby/gems/1.9.1/gems/glib2-1.2.1/lib/mkmf-gnome2.rb:542:in `required_pkg_config_package'
  from extconf.rb:58:in `<main>'

Googling around has provided some clues-- this Homebrew issue suggests that libxml-2.0.pc simply doesn't exist on OS X Mountain Lion. I can confirm that it is not in /usr/lib/pkgconfig on my system. But I know that libxml-2.0 is installed on my system (it comes with Mountain Lion). I have also installed an alternative version via Homebrew. As well as a version of pkg-config via Homebrew, which actually creates the libxml-2.0.pc file and puts it in /usr/local/Cellar/libxml2/2.9.0/lib/pkgconfig/libxml-2.0.pc. How can I make pkg-config look in this directory for the pc file?

[myokoym]

I think that the following is a workaround:

% PKG_CONFIG_PATH=/usr/local/Cellar/libxml2/2.9.0/lib/pkgconfig gem install gtksourceview2

or

% PKG_CONFIG_PATH=/usr/local/opt/libxml2/lib/pkgconfig gem install gtksourceview2

[cosmo0920]

Homebrew provides keg-only feature.
This feature is as follows:

It means the formula is installed only into the Cellar, it is not linked into /usr/local.
homebrew FAQ

And libxml2 Formula is marked keg-only. see: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/libxml2.rb
So, it seems that this issue is caused by homebrew.

[kou]

No response.