[CMake] if(string MATCHES regex) question

[Brad King]

Eric Noulard wrote:
>> If so, I still don't quite get the usage pattern in, e.g.,
>> CheckIncludeFile.cmake. What are they trying to check?
> 
> Better ask the author of this file :-)

This was a hack left from before the 'DEFINED' option was
available from the if() command.  The code

   if(VAR MATCHES "^VAR$")

is almost equivalent to

   if(NOT DEFINED VAR)

because the if() command replaces VAR with ${VAR} implicitly
when VAR is defined (but '^VAR$' is not defined and does not
get replaced).  The implicit conversion is left from *very*
early CMake when there was no ${} syntax.

-Brad