Examples: OpenGL2/3 Makefile; Added -lGL to Linux build

examples/opengl3_example/Makefile

@@ -20,7 +20,7 @@ UNAME_S := $(shell uname -s)
 
 ifeq ($(UNAME_S), Linux) #LINUX
 	ECHO_MESSAGE = "Linux"
-	LIBS = `pkg-config --static --libs glfw3`
+	LIBS = -lGL `pkg-config --static --libs glfw3`
 
 	CXXFLAGS = -I../../ -I../libs/gl3w `pkg-config --cflags glfw3`
 	CXXFLAGS += -Wall -Wformat
@@ -30,8 +30,7 @@ endif
 ifeq ($(UNAME_S), Darwin) #APPLE
 	ECHO_MESSAGE = "Mac OS X"
 	LIBS = -framework OpenGL -framework Cocoa -framework IOKit -framework CoreVideo
-	LIBS += -L/usr/local/lib
-	LIBS += -lglfw3
+	LIBS += -L/usr/local/lib -lglfw3
 
 	CXXFLAGS = -I../../ -I../libs/gl3w -I/usr/local/Cellar/glew/1.10.0/include -I/usr/local/include
 	CXXFLAGS += -Wall -Wformat

examples/opengl_example/Makefile

@@ -19,7 +19,7 @@ UNAME_S := $(shell uname -s)
 
 ifeq ($(UNAME_S), Linux) #LINUX
 	ECHO_MESSAGE = "Linux"
-	LIBS = `pkg-config --static --libs glfw3`
+	LIBS = -lGL `pkg-config --static --libs glfw3`
 
 	CXXFLAGS = -I../../ `pkg-config --cflags glfw3`
 	CXXFLAGS += -Wall -Wformat
@@ -29,8 +29,7 @@ endif
 ifeq ($(UNAME_S), Darwin) #APPLE
 	ECHO_MESSAGE = "Mac OS X"
 	LIBS = -framework OpenGL -framework Cocoa -framework IOKit -framework CoreVideo
-	LIBS += -L/usr/local/lib
-	LIBS += -lglfw3
+	LIBS += -L/usr/local/lib -lglfw3
 
 	CXXFLAGS = -I../../ -I/usr/local/include
 	CXXFLAGS += -Wall -Wformat