weird error when installing mechainze gem on mac os

I'm trying to install the mechainze gem on my mac os, however when I typed gem install mechanize, after a few seconds, it poped out:

Building native extensions.  This could take a while...
ERROR:  Error installing mechanize:
    ERROR: Failed to build gem native extension.

    /Users/Tim/.rvm/rubies/ruby-1.9.3-p545/bin/ruby extconf.rb
checking for main() in -lstdc++... yes
checking for ruby/encoding.h... yes
creating Makefile

make  clean

make
compiling unf.cc
g++: error: unrecognized command line option '-Wshorten-64-to-32'
make: *** [unf.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/Tim/.rvm/gems/ruby-1.9.3-p545@global/gems/unf_ext-0.0.6 for inspection.
Results logged to /Users/Tim/.rvm/gems/ruby-1.9.3-p545@global/extensions/x86_64-darwin-12/1.9.1/unf_ext-0.0.6/gem_make.out

I have no idea why this happened and is there anyone willing to help?

Thanks!

PS: I found out that actually a long time ago I set the default compiler to g++ instead of clang. Does this matter?

Solution

Actually this is indeed caused by my switching the default compiler to gcc4.7. After I switched back(command: sudo port select --set gcc llvm-gcc42), it went well smoothly.