Fix removal of symlinks when uninstalling

When build shared library is ON, these symlinks were installed but not
removed when uninstalling :
  libglfw.so -> libglfw.so.3
  libglfw.so.3 -> libglfw.so.3.0

cmake_uninstall.cmake.in

@@ -15,6 +15,15 @@ FOREACH(file ${files})
     IF(NOT "${rm_retval}" STREQUAL 0)
       MESSAGE(FATAL_ERROR "Problem when removing \"$ENV{DESTDIR}${file}\"")
     ENDIF(NOT "${rm_retval}" STREQUAL 0)
+  ELSEIF(IS_SYMLINK "$ENV{DESTDIR}${file}")
+    EXEC_PROGRAM(
+      "@CMAKE_COMMAND@" ARGS "-E remove \"$ENV{DESTDIR}${file}\""
+      OUTPUT_VARIABLE rm_out
+      RETURN_VALUE rm_retval
+      )
+    IF(NOT "${rm_retval}" STREQUAL 0)
+      MESSAGE(FATAL_ERROR "Problem when removing symlink \"$ENV{DESTDIR}${file}\"")
+    ENDIF(NOT "${rm_retval}" STREQUAL 0)
   ELSE(EXISTS "$ENV{DESTDIR}${file}")
     MESSAGE(STATUS "File \"$ENV{DESTDIR}${file}\" does not exist.")
   ENDIF(EXISTS "$ENV{DESTDIR}${file}")