M2 under Snow Leopard

in

	libraries/gc/build/gc-7.1/mach_dep.c

include the file <sys/ucontext.h> instead of <ucontext.h>.

(see:

http://duriansoftware.com/joe/PSA:-avoiding-the-%22ucontext-routines-are-deprecated%22-error-on-Mac-OS-X-Snow-Leopard.html

)