The unixy way of doing things is this:

clean:
    rm -f glversion.txt glversion.exe glversion.o
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The -f option tells rm (among other things) that it's not an error if some of the files don't exist. Maybe there's a similar option for del?

Another way is it let the command fail, and ignore the failure by prepending a dash to the command:

clean:
   - rm glversion.txt glversion.exe glversion.o
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This way rm will fail if some of these files don't exist, but make will ignore that error. Again I don't know if the various make dialects understand that.

To me it seems overkill to both add the leading dash and have the if exists in the makefile.