Fix C99 strict compatibility: don't use void * for function pointers

Fix C99 strict compatibility: don't use void * for function pointers

compiling public headers with --std=x99 -pedantic shows:

warning: ISO C forbids conversion of object pointer to function pointer type

Use "void (*func)(void)" to represent a generic function pointer rather
than "void *".

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>