After a make distclean I'm seeing the following output during a build of default.  It is possible my 2.7 install is borked, but I don't understand why python is apparently being called during a gcc step.  The build seems to succeed.

gcc -pthread -c  -g -O0 -Wall -Wstrict-prototypes    -I. -IInclude -I./Include    -DPy_BUILD_CORE \
      -DHGVERSION="\"`LC_ALL=C hg id -i .`\"" \
      -DHGTAG="\"`LC_ALL=C hg id -t .`\"" \
      -DHGBRANCH="\"`LC_ALL=C hg id -b .`\"" \
      -o Modules/getbuildinfo.o ./Modules/getbuildinfo.c
Traceback (most recent call last):
  File "/usr/lib/python2.7/site.py", line 62, in <module>
    import os
  File "/usr/lib/python2.7/os.py", line 398, in <module>
    import UserDict
  File "/usr/lib/python2.7/UserDict.py", line 83, in <module>
    import _abcoll
  File "/usr/lib/python2.7/_abcoll.py", line 11, in <module>
    from abc import ABCMeta, abstractmethod
ImportError: cannot import name ABCMeta
Traceback (most recent call last):
  File "/usr/lib/python2.7/site.py", line 62, in <module>
    import os
  File "/usr/lib/python2.7/os.py", line 398, in <module>
    import UserDict
  File "/usr/lib/python2.7/UserDict.py", line 83, in <module>
    import _abcoll
  File "/usr/lib/python2.7/_abcoll.py", line 11, in <module>
    from abc import ABCMeta, abstractmethod
ImportError: cannot import name ABCMeta
Traceback (most recent call last):
  File "/usr/lib/python2.7/site.py", line 62, in <module>
    import os
  File "/usr/lib/python2.7/os.py", line 398, in <module>
    import UserDict
  File "/usr/lib/python2.7/UserDict.py", line 83, in <module>
    import _abcoll
  File "/usr/lib/python2.7/_abcoll.py", line 11, in <module>
    from abc import ABCMeta, abstractmethod
ImportError: cannot import name ABCMeta

It's being called for hg:

   -DHGVERSION="\"`LC_ALL=C hg id -i .`\"" ...

As far as I can tell my python2.7 installation is fine and bug free.  hg is working fine when called from the command line.

It turns out that I had a stray abc.py file in my current working directory as the result of some previous tests of module-load-order rules.  That by itself wouldn't have triggered the problem, but in addition, I have PYTHONPATH set in my environment because I have the hg whitespace hook installed.  Apparently the build process turns a non-empty PYTHONPATH into a null entry in the final computed PYTHONPATH it uses when hg is called, and this results in anything located in the CWD being loaded.

I don't think this rises quite to the level of a security issue, since it applies only at build time, but it certainly seems like a bug.