Silence MSVC warnings about redefinition of isnan

Some versions of perl.h define isnan when the compiler is MSVC. To avoid
warnings about this, undefine the symbol before including perl.h and
re-add the definition afterwards if it wasn't recreated.

Discussion: https://postgr.es/m/caf0568e-3c1f-07fd-6914-d903f22560f2@2ndQuadrant.com

src/pl/plperl/plperl.h

@@ -40,10 +40,15 @@
 
 /*
  * ActivePerl 5.18 and later are MinGW-built, and their headers use GCC's
- * __inline__.  Translate to something MSVC recognizes.
+ * __inline__.  Translate to something MSVC recognizes. Also, perl.h sometimes
+ * defines isnan, so undefine it here and put back the definition later if
+ * perl.h doesn't.
  */
 #ifdef _MSC_VER
 #define __inline__ inline
+#ifdef isname
+#undef isnan
+#endif
 #endif
 
 /*
@@ -135,6 +140,13 @@
 #define vprintf			pg_vprintf
 #define printf(...)		pg_printf(__VA_ARGS__)
 
+/* put back the definition of isnan if needed */
+#ifdef _MSC_VER
+#ifndef isnan
+#define isnan(x) _isnan(x)
+#endif
+#endif
+
 /* perl version and platform portability */
 #define NEED_eval_pv
 #define NEED_newRV_noinc