depends: fix a static qt5 crash when using certain versions of libxcb

See here for background: https://bugreports.qt.io/browse/QTBUG-34748

libxcb temporarily had an abi breakage which caused crashes when qt was
compiled against a non-compatible version. Building qt with -qt-xcb should have
shielded us from this issue, except that incompatible headers were used when
building qt's wrapper.

Make sure those headers aren't picked up by qt's build.

Details:

qt's build adds a wrapper around the xcb libs when -qt-xcb is used. This is
done to avoid having to link to a handful of different libs, which may not be
api/abi stable. This build depends on include-order, so that its files are
found before the real libxcb headers.

Our build (for other reasons related to qt's complicated build-system) injects
our prefix into CXXFLAGS. Because libxcb is found in this path, that reverses
the include-order, negating the purpose of the wrapper.

To fix, libxcb's includes are simply moved to a subdir. pkg-config ensures that
they're still found properly when needed.

To make things even more interesting, this behavior in qt's .pro files is broken:
INCLUDEPATH += $$QMAKE_CFLAGS_XCB

The INCLUDEPATH variable is processed by qmake which automatically prefixes each
entry with "-I". The QMAKE_CFLAGS_XCB variable comes from pkg-config and
already contains -I, making the path look like "-I-I/path/to/xcb/headers".

To work around that, CFLAGS/CXXFLAGS are used here rather than INCLUDEPATH.

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