[CMake] global variable

[John Biddiscombe]

> You'll be surprised how rarely this is really needed if the directory 
> structure is organized properly.  However, there should still be an 
> easier way to do it for the times it is needed than using the file 
> write/read trick.  When designing the ADD_SUBDIRECTORY command we 
> considered adding an optional argument that might work like this:

ADD_SUBDIRECTORY(foo EXPORT_VARIABLES FOO_VAR1 FOO_VAR2)

One very nice use of the sub_dir read/write file trick that I've used is...
For every module that gets wrapped in TCL, the binary path to the 
generated pkgindex file is appended to a variable, when complete, a 
configure step creates a complete tcl path setup which is really useful.
Please allow subdirts to pass info back to the parent

I've added this as a feature request in bug tracker, but can't see it 
anywhere.

JB