Don't build libxml2.dll against debugging runtime for --enable-debug

Passing the debug=yes option to libxml2's configure.js causes it to be
built against the debugging runtime, which is not what we want if we
just have used --enable-debug. It is with --enable-dbgutil that we
want to use the debugging runtime. So look at $(dbgutil), not
$(debug).

libxml2/makefile.mk

@@ -95,7 +95,7 @@ BUILD_DIR=$(CONFIGURE_DIR)
 CONFIGURE_DIR=win32
 CONFIGURE_ACTION=cscript configure.js
 CONFIGURE_FLAGS=iconv=no sax1=yes
-.IF "$(debug)"!=""
+.IF "$(dbgutil)"!=""
 CONFIGURE_FLAGS+=debug=yes
 .ENDIF
 BUILD_ACTION=nmake