cmake: Use CMAKE_DL_LIBS instead of hardcoding "dl"
cmake: Use find_package to find X11 libraries
cmake: Use find_package to find pthreads
vogleditor: Cleanup cmake libraries

pthread is not directly used here (used by voglcore), so no need to
specify it here. X11 is duplicated.
Merge pull request #12 from andrewkww/cmake_fixes

Use cmake to find dl, X11, and pthreads