`make` should not run `coq` and `cc` when there's nothing to be done

[JasonGross]

Fiat-Crypto's start-up make is slow because make bedrock2_ex; make compiler_noex runs

COQFLAGS="-Q src/bedrock2 bedrock2 -Q src/bedrock2Examples bedrock2Examples  -Q /home/jgross/Documents/GitHub/fiat-crypto/rupicola/bedrock2/deps/coqutil/src/coqutil coqutil " ../etc/bytedump.py bedrock2.PrintListByte.allBytes > special/BytedumpTest.out.tmp
COQFLAGS="-Q src/bedrock2 bedrock2 -Q src/bedrock2Examples bedrock2Examples  -Q /home/jgross/Documents/GitHub/fiat-crypto/rupicola/bedrock2/deps/coqutil/src/coqutil coqutil " ../etc/bytedump.py bedrock2.ToCStringExprTypecheckingTest.test > special/TypecheckExprToCString.c
hexdump < /dev/null && \
        hexdump -C special/BytedumpTest.golden.bin > special/BytedumpTest.golden.hex && \
        hexdump -C special/BytedumpTest.out.tmp > special/BytedumpTest.out.hex && \
        diff -u special/BytedumpTest.golden.hex special/BytedumpTest.out.hex && \
        rm special/BytedumpTest.golden.hex special/BytedumpTest.out.hex || true
diff -u special/BytedumpTest.golden.bin special/BytedumpTest.out.tmp
mv special/BytedumpTest.out.tmp special/BytedumpTest.out
COQFLAGS="-Q src/bedrock2 bedrock2 -Q src/bedrock2Examples bedrock2Examples  -Q /home/jgross/Documents/GitHub/fiat-crypto/rupicola/bedrock2/deps/coqutil/src/coqutil coqutil " ../etc/bytedump.py bedrock2.ToCStringStackallocLoopTest.main_cbytes > special/stackloop.c
COQFLAGS="-Q src/bedrock2 bedrock2 -Q src/bedrock2Examples bedrock2Examples  -Q /home/jgross/Documents/GitHub/fiat-crypto/rupicola/bedrock2/deps/coqutil/src/coqutil coqutil " ../etc/bytedump.py bedrock2Examples.stackalloc.stacknondet_c > special/stacknondet.c
cc -fsyntax-only special/TypecheckExprToCString.c
cc -O0 special/stackloop.c -o special/stackloop
special/stackloop
cc special/stacknondet.c -o special/stacknondet
special/stacknondet

twice, even if it's already been run.