Currently, there are a number of compiler warnings generated when
building the addons on Linux, for example:

make[1]: Entering directory '/node/test/addons/zlib-binding/build'
  CXX(target) Release/obj.target/binding/binding.o
  SOLINK_MODULE(target) Release/obj.target/binding.node
  COPY Release/binding.node
make[1]: Leaving directory '/node/test/addons/zlib-binding/build'
In file included from ../binding.cc:1:
/node/src/node.h:515:51: warning:
cast between incompatible function types from
'void (*)(v8::Local<v8::Object>,
          v8::Local<v8::Value>,
          v8::Local<v8::Context>)' to
'node::addon_context_register_func' {aka
'void (*)(v8::Local<v8::Object>,
          v8::Local<v8::Value>,
          v8::Local<v8::Context>,
          void*)'} [-Wcast-function-type]
(node::addon_context_register_func) (regfunc), \
					   ^
/node/src/node.h:533:3:
note: in expansion of macro 'NODE_MODULE_CONTEXT_AWARE_X'
   NODE_MODULE_CONTEXT_AWARE_X(modname, regfunc, NULL, 0)
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
../binding.cc:58:1:
note: in expansion of macro 'NODE_MODULE_CONTEXT_AWARE'
 NODE_MODULE_CONTEXT_AWARE(NODE_GYP_MODULE_NAME, Initialize)
 ^~~~~~~~~~~~~~~~~~~~~~~~~

This commit adds the flag -Wno-cast-function-type to suppress these
warnings. With this change the warnings are not displayed anymore and
the output matches that of osx when running
'make -j8 test/addons/.buildstamp'.