message: Add operating system name macros
diff: #cmakedefine GPOS_32BIT 1
  #cmakedefine GPOS_64BIT 1
  +// These come from CMAKE_SYSTEM_NAME
  +#cmakedefine GPOS_Linux
  +#cmakedefine GPOS_Darwin
  +#cmakedefine GPOS_SunOS
  +
  // Atomics, detected by cmake/FindAtomics.cmake
  #cmakedefine GPOS_GCC_FETCH_ADD_32
  #cmakedefine GPOS_GCC_FETCH_ADD_64
  