Clean up <stdbool.h> reference in meson.build.

Commit bc5a4dfc accidentally left a check for <stdbool.h> in
meson.build's header_checks.  Synchronize with configure, which no
longer defines HAVE_STDBOOL_H.

There is still a reference to <stdbool.h> in an earlier test to see if
we need -std=c99 to get C99 features, like autoconf 2.69's
AC_PROG_CC_C99.  (Therefore the test remove by this commit was
tautological since day one: you'd have copped "C compiler does not
support C99" before making it this far.)

Back-patch to 16, where meson begins.

meson.build

@@ -2258,7 +2258,6 @@ header_checks = [
   'ifaddrs.h',
   'langinfo.h',
   'mbarrier.h',
-  'stdbool.h',
   'strings.h',
   'sys/epoll.h',
   'sys/event.h',