How do I determine if my R installation on OS X has the “--enable-R-shlib” option enabled?

I've installed R on my OS X machine via the .pkg method. I'm trying to get a Ruby gem called RSRuby to work and though it installed correctly, it's throwing an error when I try to invoke the gem.

Fatal error: R home directory is not defined

The RSRuby documentation states that R should be installed with the option --enable-R-shlib. But the R documentation states that the Mac OS X installation does this by default. How can you verify that this option is enabled on an installation if you don't know it was explicitly called during installation?

I'm chasing down why RSRuby can't find R home directory and this seems like the most likely problem.

Per RSRuby documentation, I copied this into my home directory

[~]$ R_HOME=/Library/Frameworks/R.framework/Resource

And from the R console, this is the output:

> R.home()
[1] "/Library/Frameworks/R.framework/Resources"

And during gem install, I ran this command:

gem install rsruby -- --with-R-dir=$R_HOME=/Library/Frameworks/R.framework/Resources

Also, these are the contents of my /Library/Frameworks/R.framework/Libraries folder:

[Libraries]$ ls
i386                    libRblas.dylib.dSYM     libgfortran.2.dylib
libR.dylib              libRblas.vecLib.dylib   libreadline.5.2.dylib
libR.dylib.dSYM         libRlapack.dylib        libreadline.dylib
libRblas.0.dylib        libRlapack.dylib.dSYM   ppc
libRblas.dylib          libgcc_s.1.dylib        x86_64