fix dl link error

fixes https://github.com/JoeyDeVries/LearnOpenGL/issues/64

CMakeLists.txt

@@ -39,6 +39,7 @@ elseif(UNIX AND NOT APPLE)
   # note that the order is important for setting the libs
   # use pkg-config --libs $(pkg-config --print-requires --print-requires-private glfw3) in a terminal to confirm
   set(LIBS ${GLFW3_LIBRARY} X11 Xrandr Xinerama Xi Xxf86vm Xcursor GL dl pthread ${ASSIMP_LIBRARY})
+  set (CMAKE_CXX_LINK_EXECUTABLE "${CMAKE_CXX_LINK_EXECUTABLE} -ldl")
 elseif(APPLE)
   INCLUDE_DIRECTORIES(/System/Library/Frameworks)
   FIND_LIBRARY(COCOA_LIBRARY Cocoa)