I can reproduce this by using either gentoo's python 2.5 or one I installed temporarily with ./configure --enable-shared --prefix $HOME/tmp/pytem, using a trivial distutils extension (I used http://dev.gentooexperimental.org/~marienz/ext.c and http://dev.gentooexperimental.org/~marienz/setup.py for testing). The relevant command that is run is:

gcc -pthread -shared build/temp.linux-i686-2.5/ext.o -L/home/marienz/tmp/pytem/lib/python2.5/config -lpython2.5 -o build/lib.linux-i686-2.5/ext.so

for the manually-configured python and something similar for gentoo's python. The code doing the adding was added by r45232 in svn. From the diff (svn di -r45231:45232 http://svn.python.org/projects/python/trunk/Lib/distutils/command/build_ext.py) with some extra context added:

-        if sys.platform[:6] == 'cygwin' or sys.platform[:6] == 'atheos':
+        if sys.platform[:6] == 'cygwin' or sys.platform[:6] == 'atheos' or \
+               (sys.platform.startswith('linux') and
+                sysconfig.get_config_var('Py_ENABLE_SHARED')):
             if string.find(sys.executable, sys.exec_prefix) != -1:
                 # building third party extensions
                 self.library_dirs.append(os.path.join(sys.prefix, "lib",
                                                      "python" + get_python_version(),
                                                      "config"))

(that is around line 188 of Lib/distutils/command/build_ext.py)

sys.platform on this host is linux2 and as far as I can tell Py_ENABLE_SHARED is true if --enable-shared is passed to configure.

If you need any more information please ask.