qt currently falls back to bundled versions of sqlite, harfbuzz, pcre,
double-conversion and xcb. This adds the appropriate dependencies and
configure arguments. A new variant adds multibyte support to pcre and
pcre2, which is required by qt.
Additionally, newer versions of gcc (starting with @8.3.0) cause build
failures. This adds a patch to fix the problem.
The changes have been tested with all versions of qt currently available
in Spack. 5.2 and 5.3 do not build for reasons that seem to be unrelated
to these changes, though.
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