[CMake] set_if_not_set

[Bill Newcomb]

My top-level CMakeLists.txt files always end up with a lot of

if(NOT_DEFINED foo)
     set(foo bar)
endif()

things to allow people to pass interesting things on the command line or 
to write their own "set this and that" CMakeLists.txt and include the 
top-level one.

It would improve readability to have set_if_not_set and friends from 
CTest.cmake available in a separate module or, better still, as language 
primitives, either as-is or like

set(IF_NOT_SET foo bar)

or maybe

set(foo bar IF_NOT_SET)

Obviously I could define set_if_not_set at the top of each top-level 
CMakeLists.txt, but the above would be nicer.

What do you think?

Thanks,
B.

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