Merge bitcoin/bitcoin#22397: build: Fix macOS Apple Silicon build with miniupnpc and libnatpmp

2445df4 build: Fix macOS Apple Silicon build with miniupnpc and libnatpmp (Hennadii Stepanov)

Pull request description:

  On master (7a49fdc) the `configure` script does not pick up Homebrew's `miniupnpc` and `libnatpmp` packages on macOS Apple Silicon:

  ```
  % ./configure --with-miniupnpc
  ...
  checking for miniupnpc/miniupnpc.h... no
  checking for miniupnpc/upnpcommands.h... no
  checking for miniupnpc/upnperrors.h... no
  ...
  checking whether to build with support for UPnP... configure: error: "UPnP requested but cannot be built. Use --without-miniupnpc."
  ```

  ```
  % ./configure --with-natpmp
  ...
  checking for natpmp.h... no
  ...
  checking whether to build with support for NAT-PMP... configure: error: NAT-PMP requested but cannot be built. Use --without-natpmp
  ```

  The preferred Homebrew [prefix for Apple Silicon](https://docs.brew.sh/Installation) 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.

ACKs for top commit:
  Zero-1729:
    re-tACK 2445df4 (re-tested on an M1 Machine running macOS 11.4).
  jarolrod:
    re-ACK 2445df4

Tree-SHA512: d623d26492f463812bf66ca519847ff4b23d517466b6c51c3caf3642a582d02e5f03ce57915742b29f01bf9bceb731a3978ef9a5fdc82e568bcb62548eda758a

Author: laanwj

configure.ac

@@ -705,6 +705,33 @@ 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