Fix Yocto cross-compiles

Fix Yocto cross-compiles

Yocto defines it's compilers in the environment complete with some arch flags.
For example:

```
CC="arm-poky-linux-gnueabi-gcc \
  -march=armv7-a \
  -mfloat-abi=hard \
  -mfpu=neon \
  -mtune=cortex-a9 \
  --sysroot=/home/ubuntu/device/build/tmp/sysroots/pico-imx6"
```

The SConstruct file would fail to find the compiler because it was calling
python's subprocess.check_output which expects the first argument of the list
to be _only_ the name of the executable.

This patch allows the SConstruct script to check the version of the compiler
even with funny environment variables. Instead of appending to the compiler
string and passing `shell=True` to subprocess, split `env['CXX']` into an array
and append `-dumpversion` to that array. Python warns against the use of
`shell=True` in subprocess calls.