Required kwarg on more compiler methods

The following compiler methods now support the required keyword argument:

• compiler.compiles()
• compiler.links()
• compiler.runs()

cc.compiles(valid, name: 'valid', required : true)
cc.links(valid, name: 'valid', required : true)
cc.run(valid, name: 'valid', required : true)

assert(not cc.compiles(valid, name: 'valid', required : opt))
assert(not cc.links(valid, name: 'valid', required : opt))
res = cc.run(valid, name: 'valid', required : opt)
assert(res.compiled())
assert(res.returncode() == 0)
assert(res.stdout() == '')
assert(res.stderr() == '')