1. To use this feature, set CC and CXX env variables to the appropriate cross compiler
2. Example, for cross compiling to ARM, use: CC=arm-linux-gnueabihf-gcc-4.9 CXX=arm-linux-gnueabihf-g++-4.9 npm install
3. You can still compile without setting CC and CXX, you can still just run npm install
Rationale:
- On OS X, a user can have /usr/include, but still not have the standard headers
- this could be because the user uninstalled packages but the /usr/include directory was left behind.
- by checking for at least stdlib.h, we stand a good chance at finding an issue much easlier
