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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<div class='path'>path: <a href='/guix/log/?id=5a1ac6138d31f5e48c273b4f9292292d76d7432a'>root</a>/<a href='/guix/log/build-aux?id=5a1ac6138d31f5e48c273b4f9292292d76d7432a'>build-aux</a>/<a href='/guix/log/build-aux/test-env.in?id=5a1ac6138d31f5e48c273b4f9292292d76d7432a'>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=5a1ac6138d31f5e48c273b4f9292292d76d7432a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></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></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-04-22 23:50:49 +0200'>2018-04-22</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=479c0e3f60f54e3c3a02f71efeb49119e20d2a96'>build: Fix possible race condition in 'test-env'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-08-28 23:51:16 +0200'>2017-08-28</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=4f024ef3181c5235ff11789e8c8f62722f974787'>build: Remove code to download the Guile bootstrap tarball.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-06-26 23:11:07 +0200'>2016-06-26</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=9dd674db017dbdc451cfd35da2dc3ce08db0726a'>daemon: Rename 'NIX_CONF_DIR' to 'GUIX_CONFIGURATION_DIRECTORY'.</a><span class='msg-avail'>...</span></td><td>David Craven</td></tr>
<tr><td><span title='2016-05-04 23:35:55 +0200'>2016-05-04</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=4d9ed58498e3acb5bad1c5b862853f72bd3312aa'>build: Preserve stderr for shell tests.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-04-15 00:59:07 +0200'>2016-04-15</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=a1097caa6a30f45661ee510c59bf0e01972cebba'>build: Move environment '.in' scripts to 'build-aux' directory.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>