As part of the initial changes for Issue13590 in 2.7.3, 3.2.3, and 3.3 (prior to release) to Distutils to support building extension modules on OS X with Xcode 4, code was added to substitute the clang compiler if the older gcc-4.2 supplied with Xcode 3 was not found.  In addition, the changes made it easier for the user to override the compiler for both the compile and linking stages: "Also as a convenience, if the user does explicitly set CC, substitute its value as the default compiler in the Distutils LDSHARED configuration variable for OS X."  This eliminated the need to construct a complete LDSHARED value.  For 2.7.4, 3.2.4, and 3.3.0, more extensive changes were made to fully support the latest releases of Xcode 4.  Unfortunately, the code to override LDSHARED if CC is set was inadvertently deleted for these later releases.  The attached patch restores the deleted override behavior and adds tests.

LGTM

New changeset ca24bc6a5a4b by Ned Deily in branch '2.7':
Issue #18080: When building a C extension module on OS X, if the compiler
http://hg.python.org/cpython/rev/ca24bc6a5a4b

New changeset 75432fb6b9af by Ned Deily in branch '3.3':
Issue #18080: When building a C extension module on OS X, if the compiler
http://hg.python.org/cpython/rev/75432fb6b9af

New changeset 0512bb1b5b8a by Ned Deily in branch 'default':
Issue #18080: merge from 3.3
http://hg.python.org/cpython/rev/0512bb1b5b8a

Committed for release with 2.7.6 and 3.3.3.