Modify verify.h to pacify gcc -Wredundant_decls.

Modify verify.h to pacify gcc -Wredundant_decls.

* lib/verify.h (GL_CONCAT, GL_CONCAT0, GL_GENSYM): New macros.
These use the prefix "GL_" since they're likely to be useful elsewhere.
We may need to break them out into a different .h file.
(__COUNTER__): Define to 0 if the compiler doesn't support it.
(verify) [!defined __cplusplus]: Use them to avoid duplicate decls
of verify_function__.