Feature/cmake cleanup (#381)

* Rename OnScreenGui module to ImGui
* Support multiple external module folders (closes #31)
* Cleaning up CMake files
  * Restructure application specification
  * Add parameter for library mode to all modules
  * Add functions to handle global variable state
* Misc/remove warnings (#383)
* Increase build timeout and do a clean rebuild every commit

modules/base/rendering/renderablesphere.cpp

@@ -31,15 +31,13 @@
 #include <openspace/util/powerscaledsphere.h>
 #include <openspace/util/updatestructures.h>
 
+#include <ghoul/glm.h>
 #include <ghoul/filesystem/filesystem.h>
 #include <ghoul/io/texture/texturereader.h>
 #include <ghoul/opengl/texture.h>
 #include <ghoul/opengl/textureunit.h>
 #include <ghoul/opengl/programobject.h>
 
-#define _USE_MATH_DEFINES
-#include <math.h>
-
 namespace {
     enum Orientation {
         Outside = 1,
@@ -228,7 +226,7 @@ void RenderableSphere::deinitialize() {
 void RenderableSphere::render(const RenderData& data, RendererTasks&) {
     glm::mat4 transform = glm::mat4(1.0);
 
-    transform = glm::rotate(transform, static_cast<float>(M_PI_2), glm::vec3(1, 0, 0));
+    transform = glm::rotate(transform, glm::half_pi<float>(), glm::vec3(1, 0, 0));
 
     // Activate shader
     using IgnoreError = ghoul::opengl::ProgramObject::IgnoreError;