CMake: Add tristate option for using system GLFW (#455)

-DWITH_SYSTEM_GLFW=ON: Link against system glfw and fail otherwise
-DWITH_SYSTEM_GLFW=OFF: Use embedded rglfw.c
-DWITH_SYSTEM_GLFW=IF_POSSIBLE: Probe for system glfw but fallback to
                                rglfw if unavailable

Also change Linux 64-bit CI build to install system glfw and use it,
so this doesn't bitrot.

Addresses #453.

utils.cmake

@@ -1,6 +1,9 @@
 # All sorts of things that we need cross project
 cmake_minimum_required(VERSION 2.8.0)
 
+set(WITH_SYSTEM_GLFW  OFF  CACHE STRING "Link raylib against system GLFW instead of embedded one")
+set_property(CACHE WITH_SYSTEM_GLFW PROPERTY STRINGS ON OFF IF_POSSIBLE)
+
 # Linking for OS X -framework options
 # Will do nothing on other OSes
 if(APPLE)
@@ -27,24 +30,59 @@ else()
     find_library(XINERAMA_LIBRARY Xinerama)
     find_library(XXF86VM_LIBRARY Xxf86vm)
     find_library(XCURSOR_LIBRARY Xcursor)
-
     include_directories(${OPENGL_INCLUDE_DIR})
 
     set(LIBS_PRIVATE m ${pthread} ${OPENGL_LIBRARIES} ${X11_LIBRARIES} ${XRANDR_LIBRARY} ${XINERAMA_LIBRARY} ${XI_LIBRARY} ${XXF86VM_LIBRARY} ${XCURSOR_LIBRARY})
   endif()
 endif()
 
+if(WITH_SYSTEM_GLFW STREQUAL "ON")
+    find_package(glfw3 3.2.1 REQUIRED)
+else(WITH_SYSTEM_GLFW STREQUAL "IF_POSSIBLE")
+    find_package(glfw3 3.2.1)
+endif()
+if (glfw3_FOUND)
+  set(LIBS_PRIVATE ${LIBS_PRIVATE} glfw)
+endif()
+
+
 if(CMAKE_SYSTEM_NAME STREQUAL Linux)
   set(LINUX TRUE)
   set(LIBS_PRIVATE dl ${LIBS_PRIVATE})
 endif()
 
+foreach(L ${LIBS_PRIVATE})
+  get_filename_component(DIR  ${L} PATH)
+  get_filename_component(LIBFILE ${L} NAME_WE)
+  STRING(REGEX REPLACE "^lib" "" FILE ${LIBFILE})
+
+  if (${L} MATCHES "[.]framework$")
+      set(FILE_OPT "-framework ${FILE}")
+      set(DIR_OPT  "-F${DIR}")
+  else()
+      set(FILE_OPT "-l${FILE}")
+      set(DIR_OPT  "-L${DIR}")
+  endif()
+
+  if ("${DIR}" STREQUAL "" OR "${DIR}" STREQUAL "${LASTDIR}")
+      set (DIR_OPT "")
+  endif()
+
+  set(LASTDIR ${DIR})
+
+  set(PKG_CONFIG_LIBS_PRIVATE ${PKG_CONFIG_LIBS_PRIVATE} ${DIR_OPT} ${FILE_OPT})
+  string (REPLACE ";" " " PKG_CONFIG_LIBS_PRIVATE "${PKG_CONFIG_LIBS_PRIVATE}")
+endforeach(L)
+
+
+
 # Do the linking for executables that are meant to link raylib
 function(link_libraries_to_executable executable)
-  # And raylib
-  target_link_libraries(${executable} raylib)
-
-  # Link the libraries
-  target_link_libraries(${executable} ${LIBS_PRIVATE})
+  # Link raylib
+  if (TARGET raylib_shared)
+    target_link_libraries(${executable} raylib_shared)
+  else()
+    target_link_libraries(${executable} raylib ${PKG_CONFIG_LIBS_PRIVATE})
+  endif()
 endfunction()