Bug#899382: qt5-qmake: cross wrapper should supply PKG_CONFIG_EXECUTABLE

[Helmut Grohne <helmut@subdivi.de>]

On Wed, May 23, 2018 at 05:14:06PM +0200, Helmut Grohne wrote:
> qtmultimedia-opensource-src fails to cross build from source, because it
> fails finding gstreamer and pulseaudio. Looking at its configure.json, I
> see that the ones that are failing are discovered using a "type":
> "pkgConfig" check. This one is implemented in the function
> qtConfLibrary_pkgConfig in
> /usr/lib/<triplet>/qt5/mkspecs/features/qt_configure.prf. It discovers
> the pkg-config to use from the function qtConfPkgConfig in the same
> file. This function does not look at the PKG_CONFIG variable, but
> insists on using the PKG_CONFIG_EXECUTABLE variable. Thus the qmake
> cross wrapper should also supply PKG_CONFIG_EXECUTABLE in addition to
> PKG_CONFIG with the same value.

Thiago Macieira suggested an alternative approach to solving this.
Rather than telling qmake to use the "right" pkg-config at runtime, we
can tell it at build time.

To that end we should build qtbase with a triplet-prefixed pkg-config.

That actually works fairly well as the Debian packaging of pkg-config
ensures that <nativetriplet>-pkg-config is a symlink to pkg-config. So
just using the triplet-prefixed pkg-config always works (native and
cross). Where should we reassign this bug to?

I think Thiago's approach is strictly better than mine, because it
ensures that qmake will always use the right pkg-config even without
passing PKG_CONFIG or PKG_CONFIG_EXECUTABLE to qmake. That should be
useful to people cross building with qmake without our cross wrapper.

Helmut