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.
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.