[NaCl SDK] Allow dsc files to omit TOOLS setting

native_client_sdk/src/libraries/gtest/library.dsc

 {
-  'TOOLS': ['bionic', 'newlib', 'glibc', 'pnacl', 'win', 'linux'],
   'SEARCH': [
     '.',
     '../../../../testing/gtest/include/gtest',
     '../../../../testing/gtest/include/gtest/internal',
     '../../../../testing/gtest/src',
   ],
   'TARGETS': [
     {
       'NAME' : 'gtest',
       'TYPE' : 'lib',
       'SOURCES' : [
         'gtest.cc',
         'gtest-death-test.cc',
         'gtest-filepath.cc',
         'gtest_main.cc',
         'gtest-port.cc',
         'gtest-printers.cc',
         'gtest-test-part.cc',
         'gtest-typed-test.cc',
       ],
       'INCLUDES': [
         # See comment below about gtest-internal-inl.h
         '$(NACL_SDK_ROOT)/include/gtest/internal',
       ],
-      'CXXFLAGS': ['-Wno-unused-const-variable'],
     }
   ],
   'HEADERS': [
     {
       'FILES': [
         'gtest-death-test.h',
         'gtest.h',
         'gtest-message.h',
         'gtest-param-test.h',
         'gtest_pred_impl.h',
@@ skipping 28 matching lines @@
       # spot.
       'FILES': [
         'gtest-internal-inl.h',
       ],
       'DEST': 'include/gtest/internal/src',
     },
   ],
   'DEST': 'src',
   'NAME': 'gtest',
 }