Use thread-local storage for context stack
Use thread-local storage for context stack

Make it possible to use libvala from multiple threads by using a
thread-local stack of CodeContext objects. Based on patch by
Andrea Del Signore, fixes bug 573041.

playya/vala,themiwi/vala,jessevdk/vala,xdave/pvala,playya/vala,frida/vala,kamanashisroy/aroop,GNOME/vala,flobrosch/vala-fixes,spidermonkdust/vala,kamanashisroy/aroop,phako/vala,severinh/vala,tjol/vala-tjol,tjol/vala-tjol,frida/vala,Distrotech/vala,Distrotech/vala,severinh/vala,rvu95/vala,rvu95/vala,Jc2k/vala,severinh/vala,frida/vala,phako/vala,jessevdk/vala,GNOME/vala,rvu95/vala,spidermonkdust/vala,flobrosch/vala-fixes,lovebug356/vala,frida/vala,kamanashisroy/aroop,Distrotech/vala,GNOME/vala,lovebug356/vala,playya/vala,GNOME/vala,themiwi/vala,Jc2k/vala,flobrosch/vala-fixes,spidermonkdust/vala,jessevdk/vala,tjol/vala-tjol,frida/vala,themiwi/vala