--- misc/nss-3.39/nspr/configure
+++ misc/build/nss-3.39/nspr/configure
@@ -7300,15 +7300,27 @@
             if test -z "$GNU_CC" -a "$MSC_VER" -ge "1700"; then
                                                 CFLAGS="$CFLAGS -arch:IA32"
             fi
 	fi
         ;;
     x86_64)
-	    $as_echo "#define _AMD64_ 1" >>confdefs.h
-
-	    USE_64=1
+	    if test -z "$USE_64"; then
+		# config.guess reports the HOST, and every Windows build host is
+		# x86_64 now, so reaching this case says nothing about what is
+		# being BUILT.  Without --enable-64bit a 32-bit product was asked
+		# for, and defining _AMD64_ for one is not merely cosmetic: the
+		# Windows 10 SDK selects the Interlocked* declarations on these
+		# macros, so an x86 compile with _AMD64_ set gets no declaration
+		# at all, calls them as cdecl, and fails to link.
+		$as_echo "#define _X86_ 1" >>confdefs.h
+
+	    else
+		$as_echo "#define _AMD64_ 1" >>confdefs.h
+
+		USE_64=1
+	    fi
 	    ;;
     ia64)
 	    $as_echo "#define _IA64_ 1" >>confdefs.h
 
 	    USE_64=1
 	    ;;