Factor an <INCLUDES> placeholder out of <FLAGS> in rule variables

Teach the Makefile and Ninja generators to substitute for an <INCLUDES>
placeholder instead of putting -I in <FLAGS>.  Update our values for

  CMAKE_<LANG>_COMPILE_OBJECT,
  CMAKE_<LANG>_CREATE_ASSEMBLY_SOURCE, and
  CMAKE_<LANG>_CREATE_PREPROCESSED_SOURCE

to place <INCLUDES> just before <FLAGS>.