I struggled with this too.  I ended up changing the softlinks (in /usr/bin) for gcc and g++ to the gcc-4.2 and g++-4.2 binaries and naming the gcc toolset.

Damien

From: Stuart Dootson [mailto:stuart.dootson@gmail.com]
To: boost-users@lists.boost.org
Sent: Sun, 08 Feb 2009 14:33:17 -0700
Subject: [Boost-users] Building on OS X

So, I've installed Xcode 3.0, followed by Xcode 3.1.2. gcc is
defaulting to 4.0, which builds Boost fine (using the configure & make
install OR bjam routes).

However, I would like to build boost with gcc 4.2, as installed with
Xcode 3.1.2 - I've not managed to find the correct incantation yet to
persuade Boost to use 4.2 vs 4.0 - help!!!!

I guess I could just alter the gcc and g++ soft links in /usr/bin, but
that seems...wrong. Assistance would be greatly appreciated.

TIA!!!
Stuart Dootson
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