6b5506a286 Fix Qt's rcc determinism for depends/gitian (Fuzzbawls)
Pull request description:
With the update to Qt 5.9 having been merged, Qt's `rcc` tool now embeds a file's last modified time in it's output. Since the build system generates temporary files for all locale translations (`*.qm` files) at build time, the resulting `qrc_bitcoin_locale.cpp` file was always being generated in a non-deterministic way.
This is a backport of https://bugreports.qt.io/browse/QTBUG-62511, which is included in Qt versions 5.11+, that allows for an environment variable (`QT_RCC_SOURCE_DATE_OVERRIDE`) to override the behavior described above. This environment variable is in turn set in the gitian descriptors, as that is where determinism is vital for release purposes.
Prior to this, the `qt_libbitcoinqt_a-qrc_bitcoin_locale.o` object file (included into `libbitcoinqt.a`) was returning a different `sha256sum` for each and every build, regardless of file contents change, thus breaking determinism in the resulting binaries.
This should fix#13731
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