Installing REE 1.8.7 with rvm on OSX Mountain Lion

I was running into the following error:

rbx:home robert$ rvm install ree
Installing Ruby Enterprise Edition from source to: /Users/robert/.rvm/rubies/ree-1.8.7-2012.02
ree-1.8.7-2012.02 - #fetching (ruby-enterprise-1.8.7-2012.02)
ree-1.8.7-2012.02 - #extracting ruby-enterprise-1.8.7-2012.02 to /Users/robert/.rvm/src/ree-1.8.7-2012.02
Applying patch /Users/robert/.rvm/patches/ree/1.8.7/tcmalloc.patch
Applying patch /Users/robert/.rvm/patches/ree/1.8.7/stdout-rouge-fix.patch
Applying patch /Users/robert/.rvm/patches/ree/1.8.7/no_sslv2.diff
Applying patch 'lib64' (located at /Users/robert/.rvm/patches/ree/lib64.patch)
ree-1.8.7-2012.02 - #installing
Error running './installer -a /Users/robert/.rvm/rubies/ree-1.8.7-2012.02 --no-tcmalloc --dont-install-useful-gems --no-dev-docs -c --disable-tcl -c --disable-tk', please read /Users/robert/.rvm/log/ree-1.8.7-2012.02/install.log
There has been an error while trying to run the ree installer. Halting the installation.

Turns out X11 wasn't being found and needed to specify the gcc I wanted it to use. This worked for me:

$ export CPPFLAGS=-I/opt/X11/include
$ CC=/usr/local/bin/gcc-4.2 rvm reinstall ree