Patch taken from Nix:
   https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/qt-5/5.11/qtwebkit.patch

diff --git a/Source/WebKit2/PlatformQt.cmake b/Source/WebKit2/PlatformQt.cmake
--- a/Source/WebKit2/PlatformQt.cmake
+++ b/Source/WebKit2/PlatformQt.cmake
@@ -261,6 +261,7 @@
 list(APPEND WebKit2_SYSTEM_INCLUDE_DIRECTORIES
     ${GLIB_INCLUDE_DIRS}
     ${GSTREAMER_INCLUDE_DIRS}
+    ${GSTREAMER_PBUTILS_INCLUDE_DIRS}
     ${Qt5Quick_INCLUDE_DIRS}
     ${Qt5Quick_PRIVATE_INCLUDE_DIRS}
     ${SQLITE_INCLUDE_DIR}

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<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
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<div class='path'>path: <a href='/guix/log/?id=42cf69c24549efd403fd944e55a9b69cb8783013'>root</a>/<a href='/guix/log/build-aux?id=42cf69c24549efd403fd944e55a9b69cb8783013'>build-aux</a>/<a href='/guix/log/build-aux/test-env.in?id=42cf69c24549efd403fd944e55a9b69cb8783013'>test-env.in</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/build-aux/test-env.in?id=42cf69c24549efd403fd944e55a9b69cb8783013&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-04-20 12:13:16 +0200'>2021-04-20</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=d95168321f4a9bf6857b598da0a183b45a868d54'>import: Remove Nix importer.</a><span class='msg-avail'>...<span class='msg-tooltip'>This importer has suffered from bitrot and no longer works with current
Nix and Nixpkgs.  See &lt;https://bugs.gnu.org/32339&gt; and
&lt;https://bugs.gnu.org/36255&gt;.

* guix/import/snix.scm, guix/scripts/import/nix.scm,
tests/snix.scm: Remove.
* Makefile.am (MODULES, SCM_TESTS): Remove them.
* guix/scripts/import.scm (importers): Remove "nix".
* build-aux/test-env.in: Remove NIXPKGS variable.
* configure.ac: Remove '--with-nixpkgs' option.
* doc/guix.texi (Invoking guix import): Remove bit about "guix import
nix".
* etc/completion/fish/guix.fish: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-10-16 22:53:40 +0200'>2019-10-16</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=a1aaca314ca94700ebe3449d6bd73522f2d243bc'>build: Remove references to 'NIX_LOCALSTATE_DIR'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* build-aux/test-env.in: Remove references to 'NIX_LOCALSTATE_DIR'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-10-16 22:53:40 +0200'>2019-10-16</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=47e99e626b46b2fc433506a822d251de68ad6241'>daemon: Remove traces of 'NIX_ROOT_FINDER'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 2e3e5d21988fc2cafb2a9eaf4b00976ea425629d.

* build-aux/test-env.in: Remove mentions of 'NIX_ROOT_FINDER'.
* nix/libstore/gc.cc (LocalStore::collectGarbage): Adjust comment
accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-02-04 00:03:30 +0100'>2019-02-04</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=a87d66f371da2a84d7bba1cae58b71c9c9af73aa'>daemon: Rename 'NIX_STATE_DIR' and 'NIX_DB_DIR' environment variables.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/22459&gt;.
Reported by Jeff Mickey &lt;j@codemac.net&gt;.

* guix/config.scm.in (%state-directory): Change NIX_STATE_DIR to
GUIX_STATE_DIRECTORY.
(%store-database-directory): Change NIX_DB_DIR to
GUIX_DATABASE_DIRECTORY.
* nix/libstore/globals.cc (Settings::processEnvironment): Likewise.
* guix/self.scm (make-config.scm): Likewise.
* build-aux/build-self.scm (make-config.scm): Likewise.
* build-aux/test-env.in: Likewise.
* tests/derivations.scm ("derivation #:leaked-env-vars"): Likewise.
* tests/guix-build.sh (GUIX_DAEMON_SOCKET): Likewise.
* tests/guix-daemon.sh (socket): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-05-13 23:16:16 +0200'>2018-05-13</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=9e7b87ddc51c640c529d445e350c3d6ef43e301d'>build: 'test-env' does not 'guix download' non-existent files.</a><span class='msg-avail'>...<span class='msg-tooltip'>Reported by uniq10.

* build-aux/test-env.in: Check whether FILE exists before invoking 'guix
download'.
</span></span></td><td>Ludovic Courtès</td></tr>