[CMake] FIND sub-command to the STRING command

[SF Markus Elfring]

> mhh, yeah, thats a good idea. Ok, I'll use this for doing a "STRING( FIND ...)" :) It was just an idea to share this patch with you, but you're right. Its easier doing this with regular expressions.

Can it be that your implementation was faster than a complete regex application 
for your use case?

Are there any remarkable differences in the corresponding execution speed and 
performance characteristics?

Regards,
Markus