build: remove duplicate -lminiupnpc linking

Having the link check in the header check loop means we get `-lminiupnpc
-lminiupnpc -lminiupnpc` on the link line. This is unnecessary, and
results in warnings, i.e:
```bash
ld: warning: ignoring duplicate libraries: '-levent', '-lminiupnpc'
ld: warning: ignoring duplicate libraries: '-levent', '-lminiupnpc'
ld: warning: ignoring duplicate libraries: '-levent', '-lminiupnpc'
```

These warnings have been occurring since the new linker released with
Xcode 15, and also came up in hebasto/bitcoin#34.

configure.ac

@@ -1440,15 +1440,13 @@ dnl Check for libminiupnpc (optional)
 if test "$use_upnp" != "no"; then
   TEMP_CPPFLAGS="$CPPFLAGS"
   CPPFLAGS="$CPPFLAGS $MINIUPNPC_CPPFLAGS"
-  AC_CHECK_HEADERS(
-    [miniupnpc/miniupnpc.h miniupnpc/upnpcommands.h miniupnpc/upnperrors.h],
-    [AC_CHECK_LIB([miniupnpc], [upnpDiscover], [MINIUPNPC_LIBS="$MINIUPNPC_LIBS -lminiupnpc"], [have_miniupnpc=no], [$MINIUPNPC_LIBS])],
-    [have_miniupnpc=no]
-  )
+  AC_CHECK_HEADERS([miniupnpc/miniupnpc.h miniupnpc/upnpcommands.h miniupnpc/upnperrors.h], [], [have_miniupnpc=no])
 
-  dnl The minimum supported miniUPnPc API version is set to 17. This excludes
-  dnl versions with known vulnerabilities.
   if test "$have_miniupnpc" != "no"; then
+    AC_CHECK_LIB([miniupnpc], [upnpDiscover], [MINIUPNPC_LIBS="$MINIUPNPC_LIBS -lminiupnpc"], [have_miniupnpc=no], [$MINIUPNPC_LIBS])
+
+    dnl The minimum supported miniUPnPc API version is set to 17. This excludes
+    dnl versions with known vulnerabilities.
     AC_MSG_CHECKING([whether miniUPnPc API version is supported])
     AC_PREPROC_IFELSE([AC_LANG_PROGRAM([[
         @%:@include <miniupnpc/miniupnpc.h>