How to use pip install where one requirement has dependency on version on gcc-4.5

I am installing an egg packaged for pip, inside my virtualenv, under Python 2.7.2. The egg has 16 requirements, one of which (pycryptopp 0.5.29) is known to fail with gcc-4.6 and hence must be compiled with 4.5. The system has both gcc-4.6 (default) and gcc-4.5 installed.

How do I configure/hack pip install to build this package specially? (or do I just temporarily kludge the link /usr/bin/gcc while installing this package)

Do I need to clean up the existing (virtualenv)/build directory where it broke, and if so how?

(I already read the pip documentation and searched SO + SU)