Bump stack to lts-6.12 and remove extra pkg from extra-deps

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

[skip ci]

See #283

Use GCC's `-MM`/`-MG` to find missing dependencies

This allows finding missing includes for `.c` files (this is important
for all the cases where we generate the includes during the build
process).

We're using GCC's `-MM` `-MG` options and iterate as long as we get
new includes. Since this would return all includes verbatim from the
`#include`, we check which ones are actually generated and what are
their final paths.

Note: this is currently applied only to `.c` files and does not (yet?)
work for `.hs` files (there are issues with things like ifdefs for
package versions that cause GCC to error out).

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Fix #282.

[skip ci]

[skip ci]

See #280.

See #278

Make generatedDependencies an Expr [FilePath]

This fixes a TODO to change the `generatedDependencies` to
use `Expr`.

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Use in-tree cabal in build.cabal.sh

As suggested by mpickering

Travis was failing because it uses `cabal install --only-dependencies`
in the .travis.yml

Partial fix of #274

This installs the in-tree Cabal into the cabal-sandbox before building
hadrian itself. This only works if the installed cabal version is < 1.24,
because I have not yet figured out how it can be done with the newly
introduced `new-build` command.

Split CompilerMode for GHC and CC

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Fix inability to find gmp.h when building concurrently

There were situations when building concurrently when we would
request `gmp.h` before it had been built (or copied).

This was occuring when we generated the list of dependents for
the c files in the `integer-gmp` folder. Thus, when generating the
dependents for this library we now require `gmp.h`.

Change the stack configuration to use the local Cabal lib

Hadrian should be built with a local Cabal from within the
GHC codebase. This makes the stack pick it up, instead of
using the released version of Cabal.

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

See #259.