NaCl: enable BUILD_STATIC by default if building with newlib toolchain.
Updated NaCl building documentation.
Updated toolchains submodule.
Initial support for Emscripten target.
Fixed endian check.

The check was done into different variable than the one that was used in
corradeConfigure.h, effectively making everything little endian.
Emscripten: assume everything is little endian.

I didn't find any suitable solution within 5 minutes of googling. Typed
array specs say that they are in big-endian, but Chrome is storing them
as little-endian. And nobody in the web industry seems to care.
Emscripten: getenv() is not available here.

Returning empty string from Directory::home().
Utility: no need to not support Directory::Flag::SkipDotAndDotDot.

The check is purely on the name and thus it works everywhere where
`dirent.h` is available.
Emscripten: everything is either directory or unknown type.
Emscripten: std::ptr_fun is not available here.

Doing it with static_cast instead. We don't need no meaningless
wrappers, no?
Use native `corrade-rc` executable when crosscompiling.

This means that native version must be installed when crosscompiling.
Emscripten: ability to run unit tests using Node.js.

If tests are build, `ctest` runs them automatically using Node.js.
Documented Emscripten building and installation.
Emscripten: double values larger than 64 bits are not supported.
Updated documentation with better feature and platform list.