build: Use target-specific VALAFLAGS rather than AM_VALAFLAGS

build: Use target-specific VALAFLAGS rather than AM_VALAFLAGS

This allows automake to parse the flags for each target and generate
appropriate rules. For example, by explicitly specifying “--vapi foo.vapi” in
foo_VALAFLAGS (not just specifying “--library foo”), dependency and clean-up
rules for foo.vapi will be generated by automake. Accordingly, this commit
also adds missing --vapi flags.

Helps: https://bugzilla.gnome.org/show_bug.cgi?id=701099