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This is ugly, but it should fix the performance problem for older
GCC versions in the short run.  I'll look into integrating the
Gnulib module for popcount, as a better fix.
See the thread starting in:
http://lists.gnu.org/archive/html/emacs-devel/2013-09/msg00474.html
(popcount_size_t_generic) [NEED_GENERIC_POPCOUNT]:
(popcount_size_t_msc) [USE_MSC_POPCOUNT]:
(popcount_size_t_gcc) [USE_GCC_POPCOUNT]:
(popcount_size_t): Use it.