Avoid redefinition of _GNU_SOURCE
Avoid redefinition of _GNU_SOURCE

* _GNU_SOURCE may have been set by the application and redefinition
  trigger warnings or error with -Werror
* Fix for 220f6a1c5

xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS