Prevent unnecessary recompilation of Semantic.Version in dev mode.

Right now, Semantic.Version is recompiled on every invocation of
`stack build`, since we marked it as `-fforce-recomp` to ensure that
all deployments are tagged appropriately for haystack. However, this
entails a good deal of wasted time during development. With some
liberal application of `CPP`, we can make this recompilation only
happen on CI, thanks to the `release` flag and passing in a compiler
flag.

To test:

* apply the patch
* `stack build semantic`, then `stack exec semantic -- -v`. It should
  print `semantic version 0.4.0 (<development>)`.
* `stack clean semantic && stack build --ghc-options=-DCOMPUTE_GIT_SHA`.
  `stack exec semantic -- -v` should then print out the correct SHA.

Though it's probably not strictly necessary, I've marked the
`semantic` and `semanticd` executables to compile with
-DCOMPUTE_GIT_SHA, just in case.

semantic.cabal

@@ -287,7 +287,7 @@ library
                      , StrictData
                      , TypeApplications
   if flag(release)
-    ghc-options:       -Wall -Werror -Wmissing-export-lists -Wcompat -Wincomplete-record-updates -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-name-shadowing -O1 -j
+    ghc-options:       -Wall -Werror -Wmissing-export-lists -Wcompat -Wincomplete-record-updates -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-name-shadowing -O1 -j -DCOMPUTE_GIT_SHA
   else
     ghc-options:       -Wall -Wmissing-export-lists -Wcompat -Wincomplete-record-updates -Wincomplete-uni-patterns -Wredundant-constraints -fno-warn-name-shadowing -O0 -j
   ghc-prof-options:    -fprof-auto
@@ -296,7 +296,7 @@ executable semantic
   hs-source-dirs:      app
   main-is:             Main.hs
   if flag(release)
-    ghc-options:       -threaded -rtsopts "-with-rtsopts=-N -A4m -n2m" -static -j -O1 -j
+    ghc-options:       -threaded -rtsopts "-with-rtsopts=-N -A4m -n2m" -static -j -O1 -j -DCOMPUTE_GIT_SHA
   else
     ghc-options:       -threaded -rtsopts "-with-rtsopts=-N -A4m -n2m" -static -j -O0 -j
   cc-options:          -DU_STATIC_IMPLEMENTATION=1

src/Semantic/Version.hs

@@ -1,4 +1,7 @@
+{-# LANGUAGE CPP #-}
+#ifdef COMPUTE_GIT_SHA
 {-# OPTIONS_GHC -fforce-recomp #-} -- So that gitHash is correct.
+#endif
 {-# LANGUAGE TemplateHaskell #-}
 module Semantic.Version
   ( buildSHA
@@ -6,12 +9,19 @@ module Semantic.Version
   ) where
 
 import Data.Version (showVersion)
+#ifdef COMPUTE_GIT_SHA
 import Development.GitRev
+#endif
 import Paths_semantic (version)
 
 -- The SHA1 hash of this build of semantic.
+-- If compiled as a development build, this will be all zeroes.
 buildSHA :: String
+#ifdef COMPUTE_GIT_SHA
 buildSHA = $(gitHash)
+#else
+buildSHA = "<development>"
+#endif
 
 -- The version string of this build of semantic.
 buildVersion :: String