rbenv_install_1_9_2_on_yosemite

rubyinstall.sh

rbenv install 1.9.2-p330
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer//usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Downloading ruby-1.9.2-p330.tar.gz...
-> http://dqw8nmjcqpjn7.cloudfront.net/23ef45fdaecc5d6c7b4e9e2d51b23817fc6aa8225a20f123f7fa98760e8b5ca9
Installing ruby-1.9.2-p330...

BUILD FAILED (OS X 10.10 using ruby-build 20140919)

Inspect or clean up the working tree at /var/folders/ns/3sn8y8fn1k5crc9l0f1_dqvr0000gp/T/ruby-build.20141021173322.89970
Results logged to /var/folders/ns/3sn8y8fn1k5crc9l0f1_dqvr0000gp/T/ruby-build.20141021173322.89970.log

Last 10 log lines:
/var/folders/ns/3sn8y8fn1k5crc9l0f1_dqvr0000gp/T/ruby-build.20141021173322.89970/ruby-1.9.2-p330 /var/folders/ns/3sn8y8fn1k5crc9l0f1_dqvr0000gp/T/ruby-build.20141021173322.89970 ~/Sites/
configure: WARNING: unrecognized options: --with-libyaml-dir, --with-readline-dir
checking build system type... i386-apple-darwin14.0.0
checking host system type... i386-apple-darwin14.0.0
checking target system type... i386-apple-darwin14.0.0
checking whether the C compiler works... no
configure: error: in `/var/folders/ns/3sn8y8fn1k5crc9l0f1_dqvr0000gp/T/ruby-build.20141021173322.89970/ruby-1.9.2-p330':
configure: error: C compiler cannot create executables
See `config.log' for more details
make: *** No targets specified and no makefile found.  Stop.

It seems that for Ruby older than 1.9.3-p547 you will need to change your compiler (gcc) to install this version.

I've tried to install a 1.9.3 and worked fine with built-in gcc:

rbenv install 1.9.3-p547
Downloading ruby-1.9.3-p547.tar.gz...
-> http://dqw8nmjcqpjn7.cloudfront.net/9ba118e4aba04c430bc4d5efb09b31a0277e101c9fd2ef3b80b9c684d7ae57a1
Installing ruby-1.9.3-p547...
Installed ruby-1.9.3-p547 to /Users/wolcanus/.rbenv/versions/1.9.3-p547

I think you can go to 1.9.3-p547 without problems, check this changelog:

• a focus has been placed on performance with file loading, File and Pathname all significantly improved
• Ruby 1.9.2 mostly fixed down the language specification for 1.9; 1.9.3 is mostly work on a 'better implementation'
• you can tune the garbage collector via environment variables (more on this in a post coming soon!)
• Ruby's license changes from dual Ruby + GPLv2 to dual Ruby + 2-clause BSD
• improved GC performance with a lazy garbage collector
• a 'better strategy' for the GIL / GVL
• test/unit supports parallel testing
• Random.rand tweaks (and rand() now accepts ranges)
• io/console, a new library in the stdlib
• 4 new encodings (so far) - cp950, cp951, UTF-16, and UTF-32
• extra String methods
• a number of tweaks to formatting strings
• Module#private_constant and Module#public_constant
• a smattering of other bits and pieces, but this is the TLDR overview!

Source: Be Prepared for Ruby 1.9.3 and 1.9.4: What’s New and What’s Different

@squiter, check this: rbenv/ruby-build#648