Created attachment 396967 [details]
Patch

Oddly, this is also needed, only for WPE

diff --git a/Source/WebKit/PlatformWPE.cmake b/Source/WebKit/PlatformWPE.cmake
index 700ff0a480d..43e93e3b07f 100644
--- a/Source/WebKit/PlatformWPE.cmake
+++ b/Source/WebKit/PlatformWPE.cmake
@@ -279,6 +279,9 @@ list(APPEND WebKit_SYSTEM_INCLUDE_DIRECTORIES
     ${GIO_UNIX_INCLUDE_DIRS}
     ${GLIB_INCLUDE_DIRS}
     ${GSTREAMER_INCLUDE_DIRS}
+    ${GSTREAMER_AUDIO_INCLUDE_DIRS}
+    ${GSTREAMER_PBUTILS_INCLUDE_DIRS}
+    ${GSTREAMER_VIDEO_INCLUDE_DIRS}
     ${LIBSECCOMP_INCLUDE_DIRS}
     ${LIBSOUP_INCLUDE_DIRS}
 )

Right:

In file included from ../../Source/WebKit/UIProcess/API/glib/WebKitWebView.cpp:36:
In file included from ../../Source/WebKit/UIProcess/API/glib/WebKitBackForwardListPrivate.h:28:
In file included from ../../Source/WebKit/UIProcess/WebBackForwardList.h:30:
In file included from ../../Source/WebKit/UIProcess/WebPageProxy.h:50:
In file included from ../../Source/WebKit/UIProcess/SuspendedPageProxy.h:32:
In file included from DerivedSources/WebKit/WebPageProxyMessagesReplies.h:27:
In file included from ../../Source/WebKit/Shared/WebPageCreationParameters.h:32:
In file included from ../../Source/WebKit/Shared/UserContentControllerParameters.h:29:
In file included from ../../Source/WebKit/Shared/WebCompiledContentRuleListData.h:31:
In file included from DerivedSources/ForwardingHeaders/WebCore/SharedBuffer.h:52:
In file included from DerivedSources/ForwardingHeaders/WebCore/GStreamerCommon.h:25:
DerivedSources/ForwardingHeaders/WebCore/GUniquePtrGStreamer.h:24:10: fatal error: 'gst/audio/audio.h' file not found

(In reply to Víctor M. Jáquez L. from comment #2)
> Oddly, this is also needed, only for WPE
> 

The issue doesn't happen there for GTK because PlatformGTK.cmake pulls in pbutils, hence -base.

Created attachment 396979 [details]
Patch

Comment on attachment 396979 [details]
Patch

I don't understand why this has suddenly started breaking, but I can confirm it fixed my GTK build here as well (no flatpak here), so informal r+ from me.

Committed r260419: <https://trac.webkit.org/changeset/260419>

<rdar://problem/62096815>