Linux: add option to use system nspr.

base/base.gypi

Index: base/base.gypi
diff --git a/base/base.gypi b/base/base.gypi
index 2ee97d3681efdd996a8b8a3b960619fafb8658c3..fd455abf2dba07f7e902a11885cf87be8efcc2c8 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -17,9 +17,17 @@
           'third_party/dmg_fp/dtoa_wrapper.cc',
           'third_party/icu/icu_utf.cc',
           'third_party/icu/icu_utf.h',
+          'third_party/nspr/prcpucfg.h',
+          'third_party/nspr/prcpucfg_freebsd.h',
+          'third_party/nspr/prcpucfg_linux.h',
+          'third_party/nspr/prcpucfg_mac.h',
+          'third_party/nspr/prcpucfg_nacl.h',
+          'third_party/nspr/prcpucfg_openbsd.h',
+          'third_party/nspr/prcpucfg_solaris.h',
+          'third_party/nspr/prcpucfg_win.h',
           'third_party/nspr/prtime.cc',
           'third_party/nspr/prtime.h',
-          'third_party/nspr/prcpucfg_linux.h',
+          'third_party/nspr/prtypes.h',
           'third_party/xdg_mime/xdgmime.h',
           'allocator/allocator_extension.cc',
           'allocator/allocator_extension.h',
@@ -727,6 +735,11 @@
           ['OS == "win" and >(nacl_untrusted_build)==1', {
               'sources/': [ ['exclude', '\\.h$'] ],
           }],
+          ['<(use_system_nspr)==1 and >(nacl_untrusted_build)==0', {
+            'sources/': [
+              ['exclude', '^third_party/nspr/'],
+            ],
+          }],
         ],
       }],
     ],