Fix testsuite failures on Darwin (aka macOS).

Fix testsuite failures on Darwin (aka macOS).

Tests of AC_{CHECK_,}HEADER_STDBOOL were failing on Darwin for two
reasons: an m4 quoting bug in tests/local.at causing Darwin sed to
throw syntax errors, and an excessively precise interpretation of how
C99 and C++11 interact.  The latter is worth mentioning in NEWS.

 * tests/local.at (_AT_DEFINES_CMP_PRUNE): Insert [] before ‘dnl’ in
   two places to keep it separate from the result of the m4_bpatsubsts
   operation that immediately precedes it.
 * lib/autoconf/headers.m4 (AC_CHECK_HEADER_STDBOOL): Allow ‘bool’,
   ‘true’, and ‘false’ not to be macros, after including stdbool.h,
   whenever __cplusplus is defined.
 * NEWS: Document change to AC_{CHECK_,}HEADER_STDBOOL.