Prevent use of backend string functions

From PostgreSQL 12, port.h forcibly redefines printf() et al to use
the versions defined by PostgreSQL (pg_printf() et al). As this
causes linking issues in build environments which build pre-Pg12
versions against Pg12's libpq, ensure relevant macros defined
in port.h are undefined.

repmgr.h

@@ -21,6 +21,37 @@
 #include <config.h>
 #endif
 
+#ifdef vsnprintf
+#undef vsnprintf
+#endif
+#ifdef snprintf
+#undef snprintf
+#endif
+#ifdef vsprintf
+#undef vsprintf
+#endif
+#ifdef sprintf
+#undef sprintf
+#endif
+#ifdef vfprintf
+#undef vfprintf
+#endif
+#ifdef fprintf
+#undef fprintf
+#endif
+#ifdef vprintf
+#undef vprintf
+#endif
+#ifdef printf
+#undef printf
+#endif
+#ifdef strerror
+#undef strerror
+#endif
+#ifdef strerror_r
+#undef strerror_r
+#endif
+
 #ifndef _REPMGR_H_
 #define _REPMGR_H_