Note:

The EXTERN macro defined by "tcl.h" and the EXTERN() macro defined by
"jmorecfg.h" are in conflict with each other. Even more, "jmorecfg.h"
will not create its own definition if it encounters a previous
definition.

Special steps are taken in "jpegtcl*.[ch]" to allow interoperation.

*	The tcl.h header comes first everywhere.

*	In jpegtclDecls.h it is not possible to separate uses of the
	two EXTERN macros. "tcl.h" is loaded first, then its EXTERN
	definition disabled. Then we can load the jpeg
	definition. After that we have to restore the EXTERN as
	defined by "tcl.h" to satisfy the stub declarations.