build/lib: Check for and include stdbool.h by default

* stdbool.h should be widely supported by now, and the 'bool' type makes
  more semantic sense than an integer type for boolean values.
diff --git a/configure.ac b/configure.ac
index 6864a29..aa84ee5 100755
--- a/configure.ac
+++ b/configure.ac
@@ -436,6 +436,7 @@
 AC_HEADER_STDC
 AC_HEADER_TIME
 AC_HEADER_SYS_WAIT
+AC_HEADER_STDBOOL
 dnl AC_TYPE_PID_T
 AC_TYPE_UID_T
 AC_TYPE_MODE_T