Hi,

The parsing of valac flags is too permissive and incorrectly matches
some input, possibly leading to garbage in DIST_COMMON -- and then,
build failures.

The problem is that parsing of valac flags uses a non-anchored pattern
and so any argument happening to be a subset of a parsed argument will
match it, e.g. `vapi` will match against `--vapi` or `--internal-vapi`.

E.g. when using the following:

	something_VALAFLAGS = --vapidir vapi --pkg foo

`--pkg` is recognized as an interesting argument (and so, added to
`DIST_COMMON`) because `vapi` (the argument for `--vapidir`) matched
against `--vapi`.

Please find attached a patch fixing the issue.

Regards,
Colomban