Use posix-compliant __linux__ (__linux not defined on BG/Q)
Use posix-compliant __linux__ (__linux not defined on BG/Q)

biddisco/CMake,jafyvilla/CMake,shuaibarshad/KAUST-CMake,shurikasa/CMake,ptoharia/CMake,shuaibarshad/KAUST-CMake,ptoharia/CMake,shurikasa/CMake,jafyvilla/CMake,shurikasa/CMake,jafyvilla/CMake,shuaibarshad/KAUST-CMake,biddisco/CMake,biddisco/CMake,shurikasa/CMake,ptoharia/CMake,shuaibarshad/KAUST-CMake,ptoharia/CMake,jafyvilla/CMake,biddisco/CMake