Doing so doesn't work if bool is a macro rather than a typedef.

Although c.h spends some effort to support configurations where bool is
a preexisting macro, help_config.c has existed this way since
2003 (b700a6), and there have not been any reports of
problems. Backpatch anyway since this is as riskless as it gets.

Discussion: 20150812084351.GD8470@awork2.anarazel.de
Backpatch: 9.0-master