std.build: catch mistake of setting output path to build dir
std.build: catch mistake of setting output path to build dir

closes #464

raulgrell/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig,Dimenus/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,Dimenus/zig,zig-lang/zig,raulgrell/zig,Dimenus/zig