build: cmake cleanup (#22251)

- Remove unused code
- Use consistent casing. Variable names such as LibLuV_LIBRARIES is
  needlessly jarring, even if the name might be technically correct.
- Use title casing for packages. find_package(unibilium) requires the
  find_module to be named "Findunibilium.cmake", which makes it harder
  to spot when scanning the files. Instead, use "Unibilium".

cmake/FindLuajit.cmake

@@ -0,0 +1,20 @@
+find_path(LUAJIT_INCLUDE_DIR luajit.h
+          PATH_SUFFIXES luajit-2.0 luajit-2.1)
+
+if(MSVC)
+  list(APPEND LUAJIT_NAMES lua51)
+elseif(MINGW)
+  list(APPEND LUAJIT_NAMES libluajit libluajit-5.1)
+else()
+  list(APPEND LUAJIT_NAMES luajit-5.1)
+endif()
+
+find_library(LUAJIT_LIBRARY NAMES ${LUAJIT_NAMES})
+
+set(LUAJIT_LIBRARIES ${LUAJIT_LIBRARY})
+set(LUAJIT_INCLUDE_DIRS ${LUAJIT_INCLUDE_DIR})
+
+find_package_handle_standard_args(Luajit DEFAULT_MSG
+                                  LUAJIT_LIBRARY LUAJIT_INCLUDE_DIR)
+
+mark_as_advanced(LUAJIT_INCLUDE_DIR LUAJIT_LIBRARY)