While building proxmark3 in an environment without `git` installed, I
noticed the following errors on stderr:
```
mkversion create test env: 'git': No such file or directory
stat: cannot statx '../README.md': No such file or directory
```
The first line "env: 'git': No such file or directory" is expected, as
git is not installed.
However, the second line "stat: cannot statx '../README.md': No such
file or directory" is a real error: we're running from top-level
directory, so the correct path for README.md is just "README.md".
CheckExecute() has a flag to tell it if a check needs opencl support or
not and can automatically disable the check if opencl checks are not
requested. Mostly copy and paste the same feature into CheckFileExist
and add it where needed.