Silence warnings about redefining popen on Mingw-w64.

Mingw-w64 headers map popen/pclose to _popen and _pclose, but we want to use
our popen wrapper rather than the Mingw-w64. #undef the Mingw's version.

src/include/port.h

@@ -301,6 +301,18 @@ extern FILE *pgwin32_fopen(const char *, const char *);
 #define		fopen(a,b) pgwin32_fopen(a,b)
 #endif
 
+/*
+ * Mingw-w64 headers #define popen and pclose to _popen and _pclose.  We want
+ * to use our popen wrapper, rather than plain _popen, so override that.  For
+ * consistency, use our version of pclose, too.
+ */
+#ifdef popen
+#undef popen
+#endif
+#ifdef pclose
+#undef pclose
+#endif
+
 /*
  * system() and popen() replacements to enclose the command in an extra
  * pair of quotes.