Patch by Wesley Emeneker.

Too many build tool chains do not properly support it, including building
and linking an executable fine that simply segfaults when you try to run
it (such as debian jessie 8.5's gcc 4.9).  On others where it does appear
to build (ubuntu 16.04's gcc 5.4) there are still test_distutils and test_gdb
failures to deal with.

We're not going to spend time attempting to maintain a complicated white list
of what does and doesn't work in our configure.ac file.

Too many build tool chains do not properly support it, including building
and linking an executable fine that simply segfaults when you try to run
it (such as debian jessie 8.5's gcc 4.9).  On others where it does appear
to build (ubuntu 16.04's gcc 5.4) there are still test_distutils and test_gdb
failures to deal with.

We're not going to spend time attempting to maintain a complicated white list
of what does and doesn't work in our configure.ac file.

The function internal_setblocking() of the socket module did not check
the return values of ioctl() and fnctl().

CID 1294328

PyModule_GetDef() can return NULL. Let's check the return value properly
like in the other five cases.

CID 1299590

Also make it clear on the contents page what chapters are about the
legacy API.

The _locale module was using old-style APIs to set numeric module
constants from macros. The new way requires less code and properly
checks for NULL.

CID 1295027

symtable_analyze() calls analyze_block() with bound=NULL. Theoretically
that NULL can be passed down to update_symbols(). update_symbols() may
deference NULL and pass it to PySet_Contains()

_PyObject_GetNewArguments() can leave args == NULL but the __newobj_ex__
branch expects args to be not-NULL.

CID 1353201

_PyGen_Finalize() checks that gen->gi_code is not NULL before it
accesses the flags of the code object. This means that the flag
could be NULL.

It passes down the generatore to gen_close() and gen_send_ex().
gen_send_ex() did not check for gen->gi_code != NULL.

CID 1297900

The module initializer of the pyexpat module failed to check
the return value of PySys_GetObject() for NULL.

CID 982779

The signal module was using old-style module initialization with
potential NULL dereferencing.

CID 1295026

In the case of an escape character, c is never read. tok_next() is
used to advance the pointer.

CID 1225097

                   documentation consistent with the code.

Issue #21201: Improves readability of multiprocessing error message from server to client for certain exceptions

Patch by Claude Paroz.

Initial patch contributed by Denver Coneybeare.

between runs given the same Grammar.txt input regardless of the hash
randomization setting.

required for PGO linking - to be a configure time error rather than
make time when --with-optimizations is enabled.  Also improve our
ability to find the llvm-profdata tool on MacOS and some Linuxes.

Original patch by Martin Panter, new policy fixes by me.