build: remove bad double negative when discovering 'git'

[muggenhor]

This automatically works because CMake's if statement considers variables which' content ends with "-NOTFOUND" to be FALSE.

The previous version however was first tried as a variable which didn't exist. As a result after that it's interpreted as a string (not a variable anymore). So it skips the "-NOTFOUND" check and only does a "string not empty" check. Which, as this is a string constant, is always true.

Of course this only fixes the check. It doesn't address the fact that the remaining code below depends on GIT_REV having some content. So maybe instead find_program should be called with the REQUIRED flag?

[hatstand]

Of course this only fixes the check. It doesn't address the fact that the remaining code below depends on GIT_REV having some content. So maybe instead find_program should be called with the REQUIRED flag?

git isn't intended to be required here, so that builds from source tars work without it.