build: Fix macOS Apple Silicon build with miniupnpc and libnatpmp

The preferred Homebrew prefix for Apple Silicon is /opt/homebrew.
Therefore, if we do not use pkg-config to detect packages, we should set
the CPPFLAGS and LDFLAGS variables for them explicitly.

Author: hebasto

configure.ac

@@ -699,6 +699,33 @@ case $host in
          if $BREW list --versions qt5 >/dev/null; then
            export PKG_CONFIG_PATH="$($BREW --prefix qt5 2>/dev/null)/lib/pkgconfig:$PKG_CONFIG_PATH"
          fi
+
+         case $host in
+           *aarch64*)
+             dnl The preferred Homebrew prefix for Apple Silicon is /opt/homebrew.
+             dnl Therefore, as we do not use pkg-config to detect miniupnpc and libnatpmp
+             dnl packages, we should set the CPPFLAGS and LDFLAGS variables for them
+             dnl explicitly.
+             if test "x$use_upnp" != xno && $BREW list --versions miniupnpc >/dev/null; then
+               miniupnpc_prefix=$($BREW --prefix miniupnpc 2>/dev/null)
+               if test "x$suppress_external_warnings" != xno; then
+                 CPPFLAGS="$CPPFLAGS -isystem $miniupnpc_prefix/include"
+               else
+                 CPPFLAGS="$CPPFLAGS -I$miniupnpc_prefix/include"
+               fi
+               LDFLAGS="$LDFLAGS -L$miniupnpc_prefix/lib"
+             fi
+             if test "x$use_natpmp" != xno && $BREW list --versions libnatpmp >/dev/null; then
+               libnatpmp_prefix=$($BREW --prefix libnatpmp 2>/dev/null)
+               if test "x$suppress_external_warnings" != xno; then
+                 CPPFLAGS="$CPPFLAGS -isystem $libnatpmp_prefix/include"
+               else
+                 CPPFLAGS="$CPPFLAGS -I$libnatpmp_prefix/include"
+               fi
+               LDFLAGS="$LDFLAGS -L$libnatpmp_prefix/lib"
+             fi
+             ;;
+         esac
        fi
      else
        case $build_os in