Further fix interaction of Perl and stdbool.h

In the case that PostgreSQL uses stdbool.h but Perl doesn't, we need to
prevent Perl from defining bool, to prevent compiler warnings about
redefinition.

src/pl/plperl/plperl.h

@@ -50,6 +50,18 @@
 #define __inline__ inline
 #endif
 
+/*
+ * Regarding bool, both PostgreSQL and Perl might use stdbool.h or not,
+ * depending on configuration.  If both agree, things are relatively harmless.
+ * If not, things get tricky.  If PostgreSQL does but Perl does not, define
+ * HAS_BOOL here so that Perl does not redefine bool; this avoids compiler
+ * warnings.  If PostgreSQL does not but Perl does, we need to undefine bool
+ * after we include the Perl headers; see below.
+ */
+#ifdef USE_STDBOOL
+#define HAS_BOOL 1
+#endif
+
 
 /*
  * Get the basic Perl API.  We use PERL_NO_GET_CONTEXT mode so that our code