if Xcode command line tools were not installed.

The former block size traded away good fit within cache lines in
order to gain faster division in deque_item().  However, compilers
are getting smarter and can now replace the slow division operation
with a fast integer multiply and right shift.  Accordingly, it makes
sense to go back to a size that lets blocks neatly fill entire
cache-lines.

GCC-4.8 and CLANG 4.0 both compute "x // 62" with something
roughly equivalent to "x * 9520900167075897609 >> 69".

library has a package, lib-tk, that cannot be imported by normal means.
Lib/test/test_tk, etc, have special code to access this package.  I will not
bother with it unless the darwin check is needed before it gets moved to
test.(test-)support.py.

in the past and might be needed again in the future

(Consolidating this check and various checks in tkinter files and moving them
to test.support and test.regrtest will be another issue.)

Patch by Tshepang Lekhonkhobe.

(Consolidating this check and various checks in tkinter files and moving them
to test.support and test.regrtest will be another issue.)

Issue #18513: Add workaround for OS X 10.8 cexp bug that leads to wrong cmath.rect(0.0,-0.0) results.

(maxlen and weakref) to the end.

The current pattern of memory access will update both the leftlink and
rightlink at the same time, so they should be positioned side-by-side
for better cache locality.

Keeping the leftlink at the front of the structure would make sense
only if the paired updates were eliminated by backporting changesets
49a9c734304d, 3555cc0ca35b, ae9ee46bd471, and 744dd749e25b.  However,
that isn't likely to happen, so we're better off with the leftlink at
the end of the structure.

Phil Webster. With that available, modify RstripExtension.py to stop deleting
null slices, which caused a file to be marked as changed when it was not.

Based on patches by Todd.Rovito and Phil Webster.