Microsoft cmd.exe has strange quoting rules. Where other shells treat
single quote as a quote character, cmd.exe seems to pass it through to
the underlying process. This breaks the vers_id compilation with
strange error messages. In practice, SCons does not need the
no-interpolation semantics that single quote provides, so switch to
double quote for compatibility with Windows. This is probably slightly
less efficient on Bash, since it will now scan the value for expansions.
However, the efficiency loss is tiny compared to overhead elsewhere in
the build.
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