[boost] [context] a couple of build fixes for Solaris (10)

First is to get the definition of mprotect() that takes void * rather than char * by defining _XPG4_2, which causes a compilation failure with Solaris Studio 12.3. Second is a workaround for a bug in their csignal header which uses the same include guard that signal.h uses, which csignal includes (leaving SIGSTKSZ undefined). Attached if of any help. Regards Luke Elliott.