bug#38405: [staging] Some Qt plugins fail to build

[Marius Bakke <mbakke@fastmail.com>]

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Hello Guix,

The 'staging' branch has been idle for a while because some Qt packages
fail to build (as of commit b60d2bfff95c0859d7814c1fe9d0940c87edc2b4):

* qtwayland: https://ci.guix.gnu.org/build/1851111/details
* qtgamepad: https://ci.guix.gnu.org/build/1946382/details
* qtwebglplugin: https://ci.guix.gnu.org/build/1830705/details

Unfortunately none of the above log files show the real issue because of
truncation: <https://bugs.gnu.org/37246>.

The problem is that for these packages specifically, the Makefile
generated by qmake references $out/lib/libQt5Core.so in LIBS (in
_addition_ to $qtbase/lib/libQt5Core.so), which in turn breaks the GCC
command line because $out/lib/libQt5Core.so obviously does not exist.

Prior to Qt 5.12, qmake generated LIBS by adding appropriate -L and -l
arguments to GCC such as "-L /gnu/store/abc123-foo/lib -l foo", whereas Qt
5.12 inserts absolute library references, i.e.
"/gnu/store/abc123-foo/lib/libfoo.so".  The former case worked because
GCC gracefully handles duplicate -L and -l arguments as long as at least
one of the '-L's are sane.

The references comes from the .prl files shipped with qtbase, which
contains entries such as:

  QMAKE_PRL_LIBS = $$[QT_INSTALL_LIBS]/libQt5Gui.so $$[QT_INSTALL_LIBS]/libQt5Core.so -lpthread /gnu/store/hhpkqcz4i8rsv3lk1iv694q0bkg2vij9-mesa-19.2.1/lib/libGL.so

QT_INSTALL_LIBS behaves differently in Guix at "configure time" (qmake)
and "build time" (when running "make", which creates other Makefiles).
At configure time it correctly resolves to the qtbase output (because we
patch qt_configure.prf to make it so).  At build time, it is repurposed
to be the installation prefix (i.e. $out/lib).

It's not clear to me why only the three mentioned packages are
affected.  My current best guess is that they fail to probe for some
(transitive?) dependency at "configure time", so no qmake cache entry is
created.  When these dependencies are resolved at "build time",
QT_INSTALL_LIBS points to the wrong location, and the build fails.

A workaround for this issue will be submitted shortly.

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