[CMake] Question on variable check in Cmake macro files

[Steve Huston]

Hi,
While trying to figure out why my check for a system header file is
not working, I was reading the CheckIncludeFile.cmake source to see
what it does. I have a question on this idiom:

 IF("${VARIABLE}" MATCHES "^${VARIABLE}$")
   ...

It seems to prevent the check for the include file from running more
than once... Is that the intention? Does this effectively check to see
if VARIABLE is in the cache?

Is there any way to force the check to run without deleting the cache
file?

Thanks,
-Steve