Linux 3.9 compat: Undefine GCC_VERSION

The mainline kernel started defining GCC_VERSION with commit
torvalds/linux@3f3f8d2f48acfd8ed3b8e6b7377935da57b27b16. Unfortunately,
LZ4 also defines this macro, but the two defintions are incompatible. We
undefine GCC_VERSION in lz4.c to handle this.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>