librealsense: add enableExamples flag, default it to false

Somewhere in the cmake machinery is a missing include/find for OpenGL::GL when BUILD_EXAMPLES=ON and BUILD_GRAPHICAL_EXAMPLES=OFF
This fixes build for all other variants than librealsense-gui.

pkgs/development/libraries/librealsense/default.nix

@@ -24,11 +24,13 @@
   cudaPackages ? { },
   enablePython ? false,
   pythonPackages ? null,
+  enableExamples ? enableGUI, # currently broken without enableGUI
   enableGUI ? false,
 }:
 
 assert cudaSupport -> (cudaPackages ? cudatoolkit && cudaPackages.cudatoolkit != null);
 assert enablePython -> pythonPackages != null;
+assert enableGUI -> enableExamples;
 
 let
   stdenv' = if cudaSupport then cudaPackages.backendStdenv else stdenv;
@@ -63,10 +65,12 @@ stdenv'.mkDerivation rec {
       pybind11
     ]
   )
+  ++ lib.optionals (enableExamples || enableGUI) [
+    glfw
+  ]
   ++ lib.optionals enableGUI [
     libgbm
     gtk3
-    glfw
     libGLU
     curl
   ];
@@ -95,7 +99,7 @@ stdenv'.mkDerivation rec {
   ];
 
   cmakeFlags = [
-    "-DBUILD_EXAMPLES=ON"
+    "-DBUILD_EXAMPLES=${lib.boolToString enableExamples}"
     "-DBUILD_GRAPHICAL_EXAMPLES=${lib.boolToString enableGUI}"
     "-DBUILD_GLSL_EXTENSIONS=${lib.boolToString enableGUI}"
     "-DCHECK_FOR_UPDATES=OFF" # activated by BUILD_GRAPHICAL_EXAMPLES, will make it download and compile libcurl
@@ -185,5 +189,6 @@ stdenv'.mkDerivation rec {
       pbsds
     ];
     platforms = lib.platforms.unix;
+    mainProgram = if enableGUI then "realsense-viewer" else "rs-enumerate-devices";
   };
 }