Now that the `libraries/dph` submodule is checked out always we need
a different way to disable building DPH to save compile-time while
developing GHC.

This commit adds a new YES/NO Make variable `BUILD_DPH` that can be used
inside mk/build.mk to control whether to build libraries/dph or not.
The default setting is `BUILD_DPH=YES` (via `mk/config.mk.in`).

This also changes `validate`'s flag `--no-dph` to explicitly disable DPH
for the current validation run.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

Test Plan: successful validates with `--fast --no-dph`

Differential Revision: https://phabricator.haskell.org/D31