/sys$common/syshlp/helplib.hlb CC, Language topics, Predefined Macros, CC$gfloat  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  When you compile using the /G_FLOAT qualifier, CC$gfloat is defined
  as 1.  When you compile without the /G_FLOAT qualifier, CC$gfloat
  is defined as 0.  The CC$gfloat macro is provided for compatiblity
  with VAX C.  The __G_FLOAT predefined macro should be used instead.