Cmake build: Use find_package_handle_standard_args() consistently
Cmake build: Search for spell checking libraries unconditionally

New option: LYX_REQUIRE_SPELLCHECK. If set and no spellchecker found
  stop the compilation
  if not set, compile in many speckeckers as possible from the set
  of 'ASPELL', 'Hunspell', 'Enchant'
Cmake build: Unify defines for spellchecker in config.h