build: Fix `AC_CHECK_HEADERS` and `AC_CHECK_LIB` for `libnatpmp` package

configure.ac

@@ -759,11 +759,11 @@ case $host in
              if test "$use_natpmp" != "no" && $BREW list --versions libnatpmp >/dev/null; then
                libnatpmp_prefix=$($BREW --prefix libnatpmp 2>/dev/null)
                if test "$suppress_external_warnings" != "no"; then
-                 CORE_CPPFLAGS="$CORE_CPPFLAGS -isystem $libnatpmp_prefix/include"
+                 NATPMP_CPPFLAGS="-isystem $libnatpmp_prefix/include"
                else
-                 CORE_CPPFLAGS="$CORE_CPPFLAGS -I$libnatpmp_prefix/include"
+                 NATPMP_CPPFLAGS="-I$libnatpmp_prefix/include"
                fi
-               CORE_LDFLAGS="$CORE_LDFLAGS -L$libnatpmp_prefix/lib"
+               NATPMP_LIBS="-L$libnatpmp_prefix/lib"
              fi
              ;;
          esac
@@ -1424,9 +1424,12 @@ fi
 
 dnl Check for libnatpmp (optional).
 if test "$use_natpmp" != "no"; then
+  TEMP_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $NATPMP_CPPFLAGS"
   AC_CHECK_HEADERS([natpmp.h],
-                   [AC_CHECK_LIB([natpmp], [initnatpmp], [NATPMP_LIBS=-lnatpmp], [have_natpmp=no])],
+                   [AC_CHECK_LIB([natpmp], [initnatpmp], [NATPMP_LIBS="$NATPMP_LIBS -lnatpmp"], [have_natpmp=no], [$NATPMP_LIBS])],
                    [have_natpmp=no])
+  CPPFLAGS="$TEMP_CPPFLAGS"
 fi
 
 if test "$build_bitcoin_wallet$build_bitcoin_cli$build_bitcoin_tx$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench" = "nonononononono"; then
@@ -1752,7 +1755,7 @@ else
     AC_MSG_RESULT($use_natpmp_default)
     AC_DEFINE_UNQUOTED([USE_NATPMP], [$natpmp_setting], [NAT-PMP support not compiled if undefined, otherwise value (0 or 1) determines default state])
     if test "$TARGET_OS" = "windows"; then
-      NATPMP_CPPFLAGS="-DSTATICLIB -DNATPMP_STATICLIB"
+      NATPMP_CPPFLAGS="$NATPMP_CPPFLAGS -DSTATICLIB -DNATPMP_STATICLIB"
     fi
   else
     AC_MSG_RESULT([no])