Add CROSS_COMPILE symbol to simplify cross-compiling

CROSS_COMPILE is a well known symbol for cross-compiling, so add it to simplify
compiling for embeded systems.

Signed-off-by: Zhengwang Ruan <rzw@meizu.com>
Merge pull request #21 from ruanzhengwang/master

Add CROSS_COMPILE symbol to simplify cross-compiling