CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
     Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, PATH_SEPARATOR, is target-dependent and determined at GCC build time.  For Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

     CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line.  This environment variable is used regardless of which language is being preprocessed.

     The remaining environment variables apply only when preprocessing the particular language indicated.  Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

     In all these variables, an empty element instructs the compiler to search its current working directory.  Empty elements can appear at the beginning or end of a path.  For instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include.

DEPENDENCIES_OUTPUT
     If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler.  System header files are ignored in the dependency output.

     The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name.  Or the value can have the form FILE TARGET, in which case the rules are written to file FILE using TARGET as the target name.

     In other words, this environment variable is equivalent to combining the options -MM and -MF (see Preprocessor Options), with an optional -MT switch too.

SUNPRO_DEPENDENCIES
     This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM.  However, the dependence on the main input file is omitted. See Preprocessor Options.