improve INSTALLDIR

configure

@@ -3789,15 +3789,11 @@ _gmake_add_switches() {
     echo "VV=@" >> "${xmake_sh_makefile}"
     echo "endif" >> "${xmake_sh_makefile}"
     echo "" >> "${xmake_sh_makefile}"
-    echo "INSTALLDIR:=" >> "${xmake_sh_makefile}"
-    echo "ifneq (\$(DESTDIR),)" >> "${xmake_sh_makefile}"
-    echo "INSTALLDIR:=\$(DESTDIR)" >> "${xmake_sh_makefile}"
-    echo "endif" >> "${xmake_sh_makefile}"
-    echo "" >> "${xmake_sh_makefile}"
     echo "ifeq (\$(PREFIX),)" >> "${xmake_sh_makefile}"
     echo "PREFIX=${_install_prefix_default}" >> "${xmake_sh_makefile}"
     echo "endif" >> "${xmake_sh_makefile}"
     echo "" >> "${xmake_sh_makefile}"
+    echo "INSTALLDIR:=\$(DESTDIR)" >> "${xmake_sh_makefile}"
     echo "ifneq (\$(PREFIX),)" >> "${xmake_sh_makefile}"
     echo "ifneq (\$(INSTALLDIR),)" >> "${xmake_sh_makefile}"
     echo "PREFIX_:=\$(patsubst /%,%,\$(PREFIX))" >> "${xmake_sh_makefile}"