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

  Compaq C automatically defines the following predefined macros
  pertaining to the format of floating-point variables.  You can use
  them to identify the format with which you are compiling your
  program:

  __D_FLOAT
  __G_FLOAT