Python-ast.c is a generated file.  As released, a Python source tarball should contain an up-to-date version that does not need to be regenerated. However, if the timestamps of the source files are not preserved, the Makefile may think it is out of date and try to regenerate it. The regeneration step requires a working Python compiler so you may run into a bootstrap issue if there is none on the system.  If you obtain the source using a Mercurial checkout, hg does not attempt to set timestamps on the source files so you are even more likely to run into problems in that case.  Usually, the solution is to do a clean checkout or tarball extraction, then manually "touch" the files that Python-ast.c is built from:

    touch Include/Python-ast.h Python/Python-ast.c

The newly-released Python 2.7.4 includes a "make touch" target in the Python Makefile just for this purpose, see Issue16004 and a number of other changes to help make this area more robust, for example, when building with a BUILDIR separate from SRCDIR.