Allow re-targeting of naclports install

Makefile

 # Copyright (c) 2012 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # Makefile
 #
 # usage: 'make [package]'
 #
 # This makefile builds all of the Native Client packages listed below
 # in $(PACKAGES). Each package has a dependency on its own sentinel
@@ skipping 33 matching lines @@
 ifeq ($(NACL_GLIBC), 1)
   NACL_TOOLCHAIN_ROOT = $(NACL_SDK_ROOT)/toolchain/$(OS_SUBDIR)_x86_glibc
   NACL_LIBC = glibc
 else
   NACL_TOOLCHAIN_ROOT = $(NACL_SDK_ROOT)/toolchain/$(OS_SUBDIR)_x86_newlib
   NACL_LIBC = newlib
 endif
 endif
 
 NACL_OUT = out
+NACLPORTS_PREFIX ?= $(NACL_TOOLCHAIN_ROOT)/$(NACL_ARCH)-nacl/usr
+
 NACL_DIRS_BASE = $(NACL_OUT)/tarballs \
                  $(NACL_OUT)/repository-$(NACL_ARCH) \
-                 $(NACL_OUT)/publish
-
-NACL_SDK_USR = $(NACL_TOOLCHAIN_ROOT)/$(NACL_ARCH)-nacl/usr
+                 $(NACL_OUT)/publish \
+                 $(NACLPORTS_PREFIX)
 
 NACL_DIRS_TO_REMOVE = $(NACL_OUT) \
-                      $(NACL_SDK_USR) \
+                      $(NACLPORTS_PREFIX) \
 
 NACL_DIRS_TO_MAKE = $(NACL_DIRS_BASE) \
-                    $(NACL_SDK_USR)/include \
-                    $(NACL_SDK_USR)/lib
+                    $(NACLPORTS_PREFIX)/include \
+                    $(NACLPORTS_PREFIX)/lib
 
 LIBRARIES = \
      libraries/nacl-mounts \
      libraries/SDL-1.2.14 \
      libraries/SDL_mixer-1.2.11 \
      libraries/SDL_image-1.2.10 \
      libraries/SDL_ttf-2.0.10 \
      libraries/gc6.8 \
      libraries/fftw-3.2.2 \
      libraries/libtommath-0.41 \
@@ skipping 209 matching lines @@
 # PNACL
 ######################################################################
 # Libraries and Examples should work for PNaCl, but if new examples
 # do not work, you can filter them here.
 WORKS_FOR_PNACL=$(LIBRARIES) $(EXAMPLES)
 
 works_for_pnacl: $(WORKS_FOR_PNACL)
 
 works_for_pnacl_list:
         @echo $(WORKS_FOR_PNACL)