The #line directive applies a specified line number and optional
  file specification to the next line of source text.  This can be
  useful for diagnostic messages.  The #line directive has the
  following forms:

       #line integer-constant <newline>
       #line integer-constant "filename" <newline>
       #line pp-tokens <newline>

  In the first two forms, the compiler gives the line following a
  #line directive the number specified by the integer constant.  The
  optional filename in quotation marks indicates the name of the
  source file that the compiler will provide in its diagnostic
  messages.  If the filename is omitted, the file name used is the
  name of the current source file or the last filename specified in a
  previous #line directive.

  In the third form, macros in the #line directive are expanded
  before it is interpreted.  This allows a macro call to expand into
  the integer-constant, filename, or both.  The resulting #line
  directive must match one of the other two forms, and is then
  processed as appropriate.