ENH: add OPTIONAL keyword to ENABLE_LANGUAGE, so it will be possible to do
something like this:

ENABLE_LANGUAGE(ASM-ATT)
IF(CMAKE_ASM-ATT_COMPILER_WORKS)
  ... do assembler stufff
ELSE(CMAKE_ASM-ATT_COMPILER_WORKS)
  ... fallback to generic C/C++
ENDIF(CMAKE_ASM-ATT_COMPILER_WORKS)

Alex