Work around missing AC_CHECK_HEADER_STDBOOL in older autoconf versions.

configure.ac

 dnl library checks.
 dnl ================================================================
 
-AC_CHECK_HEADER_STDBOOL
-
-AC_CHECK_HEADERS([fcntl.h])
-AC_CHECK_HEADERS([getopt.h])
-AC_CHECK_HEADERS([locale.h])
-AC_CHECK_HEADERS([stddef.h])
-AC_CHECK_HEADERS([sys/time.h])
-AC_CHECK_HEADERS([wchar.h])
-AC_CHECK_HEADERS([wctype.h])
+AC_CHECK_HEADERS([fcntl.h])    dnl POSIX
+AC_CHECK_HEADERS([getopt.h])   dnl POSIX
+AC_CHECK_HEADERS([locale.h])   dnl C89
+AC_CHECK_HEADERS([stddef.h])   dnl C89
+AC_CHECK_HEADERS([stdbool.h])  dnl C99
+AC_CHECK_HEADERS([sys/time.h]) dnl POSIX
+AC_CHECK_HEADERS([wchar.h])    dnl C89
+AC_CHECK_HEADERS([wctype.h])   dnl C89
 
 AC_TYPE_SIZE_T
 AC_TYPE_SSIZE_T