make-fonts.py: Use SOURCE_DATE_EPOCH if available

Gioelerequested to merge
gioele/xolonium:reproducibility into master

Currently Xolonium cannot be built in a bit-for-bit reproducible way because it embeds the current timestamp into the generated files. The Reproducible Builds projects suggests using the value of the environment variable SOURCE_DATE_EPOCH if it is available. This small patch (currently used by the Debian package fonts-xolonium) implements such logic in make-fonts.py.

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