Postgres Windows build system doesn't work with python installed in Program Files

Collegues,

Accidently I've come over minor bug in the Mkvcbuild.pm.
It happens, that it doesn't tolerate spaces in the $config->{python}
path, because it want to call python in order to find out version,
prefix and so on, and doesn't properly quote command.

Fix is very simple, see attach.

Patch is made against REL_12_STABLE, but probably applicable to other
versions as well.
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