Build executables conditionally on features (#41)

* Build executables conditionally on features

* Put away examples under a flag

cabal.project

@@ -1,3 +1,3 @@
 packages: *.cabal
 package dear-imgui
-  flags: +sdl2 +glfw +opengl2 +opengl3 +vulkan
+  flags: +sdl2 +glfw +opengl2 +opengl3 +vulkan +examples

dear-imgui.cabal

@@ -45,6 +45,14 @@ flag glfw
   manual:
     True
 
+flag examples
+  description:
+    Build executable examples.
+  default:
+    False
+  manual:
+    True
+
 common common
   build-depends:
       base
@@ -204,6 +212,8 @@ executable test
   default-language: Haskell2010
   build-depends: base, sdl2, gl, dear-imgui
   ghc-options: -Wall
+  if (!flag(examples) || !flag(sdl) || !flag(opengl2))
+    buildable: False
 
 executable glfw
   main-is: Main.hs
@@ -211,12 +221,16 @@ executable glfw
   default-language: Haskell2010
   build-depends: base, GLFW-b, gl, dear-imgui, managed
   ghc-options: -Wall
+  if (!flag(examples) || !flag(glfw) || !flag(opengl2))
+    buildable: False
 
 executable readme
   import: common
   main-is: Readme.hs
   hs-source-dirs: examples
   build-depends: sdl2, gl, dear-imgui, managed
+  if (!flag(examples) || !flag(sdl) || !flag(opengl2))
+    buildable: False
 
 executable vulkan
   import: common
@@ -251,3 +265,5 @@ executable vulkan
     , vulkan-utils
        ^>= 0.4.1
   ghc-options: -Wall
+  if (!flag(examples) || !flag(sdl) || !flag(vulkan))
+    buildable: False