Build Fails when filename has dashes

[billywhizz]

Steps to reproduce

echo "just.print('hello')" > bench-parse.js
just build bench-parse.js --clean --static

result

clean bench-parse rm -f *.o
rm -f bench-parse
complete in 0.01 sec
build bench-parse 0.0.21 (just.js) gcc builtins.S -c -o builtins.o
builtins.S: Assembler messages:
builtins.S:81: Error: junk at end of line, first unrecognised character is `-'
builtins.S:82: Error: no such instruction: `_binary_bench-parse_js_start:'
builtins.S:84: Error: junk at end of line, first unrecognised character is `-'
builtins.S:85: Error: no such instruction: `_binary_bench-parse_js_end:'
Makefile:72: recipe for target 'builtins.o' failed
make: *** [builtins.o] Error 1
failed in 0.03 sec

[midnqp]

@billywhizz Let the user use - in filename, but internally could we replace - with _ only for the sake of Assembly syntax?