Initial cross compilion support

Finally released the changes I mention on the following issue https://github.com/cjdelisle/cjdns/issues/408 so far I'm able to cross compile successfully, for example to compile for Beaglebone Black using:
```
CROSS_COMPILE=arm-linux-gnueabihf- CROSS_CFLAGS="-O3 -mcpu=cortex-a8 -mfpu=vfp -ftree-vectorize -mfloat-abi=hard -ffast-math -fsingle-precision-constant -g" ./cross-do
```
However a binary generated with NEON does not work, see issue #408 for the problem:
```
CROSS_COMPILE=arm-linux-gnueabihf- CROSS_CFLAGS="-O3 -mcpu=cortex-a8 -mfpu=neon -ftree-vectorize -mfloat-abi=hard -ffast-math -fsingle-precision-constant -g" ./cross-do
```
The toolchain I use for crosspilation from debian or ubuntu I used [linaro](https://launchpad.net/linaro-toolchain-binaries/trunk/2013.10)