RTS: address warnings and enable -Werror

[ckoparkar]

Following #246, this PR cleans up warnings in the RTS.

[ulysses4ever]

Terrific!

[ulysses4ever]

Interestingly, it now fails for me in the nix-shell:

[nix-shell:/mnt/data/artem/Dev/gibbon/gibbon-2]$ $g --run --packed --parallel sumtree.hs
codegen: C RTS could not be compiled: gcc -Wall -Wextra -Wpedantic -Wshadow -Werror -std=gnu11 -flto -D_GIBBON_VERBOSITY=1 -D_GIBBON_GENGC=0 -fcilkplus -D_GIBBON_PARALLEL -D_GIBBON_EAGER_PROMOTION=1 -D_GIBBON_SIMPLE_WRITE_BARRIER=0 -O3  -flto  -c -o /mnt/data/artem/Dev/gibbon/gibbon-2/gibbon-rts/rts-c/gibbon_rts.o /mnt/data/artem/Dev/gibbon/gibbon-2/gibbon-rts/rts-c/gibbon_rts.c

In file included from /mnt/data/artem/Dev/gibbon/gibbon-2/gibbon-rts/rts-c/gibbon_rts.c:4:0:
/mnt/data/artem/Dev/gibbon/gibbon-2/gibbon-rts/rts-c/gibbon_rts.h: In function 'gib_get_thread_id':
/mnt/data/artem/Dev/gibbon/gibbon-2/gibbon-rts/rts-c/gibbon_rts.h:358:12: error: implicit declaration of function '__cilkrts_get_worker_number' [-Werror=implicit-function-declaration]
     return __cilkrts_get_worker_number();
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~

I can fix it by adding the cilk #include to the gibbon_rts.h but I wonder why it fails only for me...

[ckoparkar]

I suppose no one built with --parallel after this commit. Some of the tests should have triggered this?

[ulysses4ever]

Right, I'm surprised none of the tests caught it. I hope to look closer into tests...

[ulysses4ever]

I looked into test-gibbon-examples.yaml, and it only has three --parallel examples, e.g.

gibbon/gibbon-compiler/tests/test-gibbon-examples.yaml

Lines 465 to 471 in d59464d

	- name: Vector.hs
	dir: examples/vectors
	test-flags: ["--parallel"]
	failing: [interp1]
	answer-file: examples/vectors/Vector.hs.ans
	# Cilk not supported on newer GCCs.
	skip: true

All of them are skipped! Because... wait for it!

Cilk not supported on newer GCCs.

It would be great to figure something out about Cilk!