vapigen.m4: add an argument for the default value
vapigen.m4: add an argument for the default value

The new argument allows people to specify a default value for enabling
Vala API generation (the default default value remains "auto").  This
is to accomodate projects, such as librsvg, which want more predictable
builds.

playya/vala,playya/vala,Distrotech/vala,spidermonkdust/vala,frida/vala,frida/vala,flobrosch/vala-fixes,frida/vala,flobrosch/vala-fixes,frida/vala,GNOME/vala,jessevdk/vala,tjol/vala-tjol,playya/vala,tjol/vala-tjol,Distrotech/vala,spidermonkdust/vala,rvu95/vala,spidermonkdust/vala,GNOME/vala,rvu95/vala,severinh/vala,GNOME/vala,severinh/vala,rvu95/vala,severinh/vala,tjol/vala-tjol,xdave/pvala,GNOME/vala,Distrotech/vala,flobrosch/vala-fixes,jessevdk/vala,frida/vala,jessevdk/vala