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=6a20289d5a38520fd79e80c256a0315c96a46347'>root</a>/<a href='/guix/log/gnu?id=6a20289d5a38520fd79e80c256a0315c96a46347'>gnu</a>/<a href='/guix/log/gnu/packages?id=6a20289d5a38520fd79e80c256a0315c96a46347'>packages</a>/<a href='/guix/log/gnu/packages/slang.scm?id=6a20289d5a38520fd79e80c256a0315c96a46347'>slang.scm</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/gnu/packages/slang.scm?id=6a20289d5a38520fd79e80c256a0315c96a46347&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2015-07-27 15:12:52 -0400'>2015-07-27</span></td><td><a href='/guix/commit/gnu/packages/slang.scm?id=cac8bc38a0cdddb4bfda7bd6db8a16af33a611a0'>gnu: Add newt.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2015-07-18 13:05:01 +0300'>2015-07-18</span></td><td><a href='/guix/commit/gnu/packages/slang.scm?id=e881752c39d455a249731a4e81e64d86f50afc34'>gnu packages: Clean up synopses and descriptions.</a><span class='msg-avail'>...</span></td><td>Alex Kost</td></tr>
<tr><td><span title='2015-03-08 01:36:35 -0500'>2015-03-08</span></td><td><a href='/guix/commit/gnu/packages/slang.scm?id=ee2a7f0789fd4fc4cf8009b9dbbfdc0dd4294478'>gnu: slang: Combine duplicate 'arguments' fields.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2015-03-07 19:23:10 -0500'>2015-03-07</span></td><td><a href='/guix/commit/gnu/packages/slang.scm?id=22e357397a76ef274b9b9466ace8bf31c8c472b4'>gnu: slang: Disable parallel build.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2015-01-21 15:16:10 +0100'>2015-01-21</span></td><td><a href='/guix/commit/gnu/packages/slang.scm?id=e9257d377a60e50b803de2789fce59270a19b801'>gnu: Add S-Lang.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>