[CMake] INSTALL DIRECTORY patterns highly confusing

[Dan Kegel]

> This turned out to work for me:
>
> install (
>   DIRECTORY ${samples_SOURCE_DIR}
>   DESTINATION "src"
>   PATTERN "*.in" EXCLUDE
>   PATTERN ".gitignore" EXCLUDE
> )

Here's one of my earlier attempts:

   PATTERN '*.in' EXCLUDE
   PATTERN 'CMakeLists.txt' EXCLUDE

Is it possible that single quotes cause silent failure?  Maybe those
could be flagged as likely errors.