Fix OSX errors and warnings

src/1.getting_started/1.1.hello_window/hello_window.cpp

@@ -75,4 +75,4 @@ void framebuffer_size_callback(GLFWwindow* window, int width, int height)
     // make sure the viewport matches the new window dimensions; note that width and 
     // height will be significantly larger than specified on retina displays.
     glViewport(0, 0, width, height);
-}
+}

src/1.getting_started/3.1.shaders_uniform/shaders_uniform.cpp

@@ -2,6 +2,7 @@
 #include <GLFW/glfw3.h>
 
 #include <iostream>
+#include <cmath>
 
 void framebuffer_size_callback(GLFWwindow* window, int width, int height);
 void processInput(GLFWwindow *window);
@@ -182,4 +183,4 @@ void framebuffer_size_callback(GLFWwindow* window, int width, int height)
     // make sure the viewport matches the new window dimensions; note that width and 
     // height will be significantly larger than specified on retina displays.
     glViewport(0, 0, width, height);
-}
+}