[CMake] Check if a command exists?

[Rui Maciel]

I have a small project which includes a couple of parsers whose lexers 
are generated by re2c.  I intended to set cmake so that it could check 
if re2c is present in the system, but after browsing through the docs 
I've ended up empty-handed.

So, is there a way to set a cmake project so that it checks if a 
tool/program/command is present in the build system?  If there is a way 
to run that check, can anyone provide an example?


Thanks in advance,
Rui Maciel