Keep galogen executable in cache

This prevents polluting the user's directory with a utility only used
at compile-time

src/OpenGL.cake

@@ -173,8 +173,14 @@ void main()\n
 
 (defun-comptime generate-gl-header (manager (& ModuleManager) module (* Module) &return bool)
   (var galogen-source-file (* (const char)) "Dependencies/galogen/galogen.cpp")
-  (var galogen-executable (* (const char))
-    (comptime-cond ('Unix "galogen") ('Windows "galogen.exe")))
+  (var galogen-executable ([] 512 char))
+  (unless (outputFilenameFromSourceFilename
+           (call-on c_str (field manager buildOutputDir))
+           "galogen"
+           ;; Add to end of file for type.
+           (comptime-cond ('Windows "exe") (true null))
+           galogen-executable (sizeof galogen-executable))
+    (return false))
 
   ;; Build galogen (GL header code generator)
   (when (fileIsMoreRecentlyModified galogen-source-file galogen-executable)