ghcide: Add flags to toggle building each executable

This package is a transient dependency of `haskell-language-server`.
However, only the library is needed. These three flags give the option for
those who do not need the executables to disable them.

`executable`:
* Toggles the `ghcide` executable
* enabled by default.

`test-exe`:
* Toggles the `ghcide-test-preprocessor` executable
* disabled by default

`bench-exe`:
* Toggles the `ghcide-bench` executable
* disabled by default

ghcide/ghcide.cabal

@@ -217,6 +217,10 @@ library
       exposed-modules:
         Development.IDE.GHC.Compat.CPP
 
+flag test-exe
+    description: Build the ghcide-test-preprocessor executable
+    default: False
+
 executable ghcide-test-preprocessor
     default-language: Haskell2010
     hs-source-dirs: test/preprocessor
@@ -225,6 +229,9 @@ executable ghcide-test-preprocessor
     build-depends:
         base == 4.*
 
+    if !flag(test-exe)
+        buildable: False
+
 benchmark benchHist
     type: exitcode-stdio-1.0
     default-language: Haskell2010
@@ -263,6 +270,10 @@ benchmark benchHist
         text,
         yaml
 
+flag executable
+    description: Build the ghcide executable
+    default: True
+
 executable ghcide
     default-language:   Haskell2010
     hs-source-dirs:     exe
@@ -322,6 +333,9 @@ executable ghcide
         TypeApplications
         ViewPatterns
 
+    if !flag(executable)
+        buildable: False
+
 test-suite ghcide-tests
     type: exitcode-stdio-1.0
     default-language: Haskell2010
@@ -404,6 +418,10 @@ test-suite ghcide-tests
         TypeApplications
         ViewPatterns
 
+flag bench-exe
+    description: Build the ghcide-bench executable
+    default: False
+
 executable ghcide-bench
     default-language: Haskell2010
     build-tool-depends:
@@ -448,3 +466,6 @@ executable ghcide-bench
         TupleSections
         TypeApplications
         ViewPatterns
+
+    if !flag(bench-exe)
+        buildable: False