build: Add $(dbus_api_checkflags) and $(dbus_api_distflags) variables
build: Add $(dbus_api_checkflags) and $(dbus_api_distflags) variables

For user-provided flags to pass to the check and dist modes of
vcs-helper.

The typical use for these is setting:
   dbus_api_checkflags = --fatal-warnings

pwithnall/dbus-deviation,pwithnall/dbus-deviation,pwithnall/dbus-deviation