webhelper: Allow building with webkit2gtk-4.1

This is the same as webkit2gtk-4.0 but using libsoup3 instead of
libsoup2, which is irrelevant for WebHelper's usage.

build/webhelper.m4

@@ -19,13 +19,22 @@ AC_DEFUN([GP_CHECK_WEBHELPER],
         fi
     fi
 
+    dnl Support both webkit2gtk 4.0 and 4.1, as the only difference is the
+    dnl libsoup version in the API, which we don't use.
+    dnl Prefer the 4.1 version, but use the 4.0 version as fallback if
+    dnl available -- yet still ask for the 4.1 if neither are available
+    webkit_package=webkit2gtk-4.1
+    PKG_CHECK_EXISTS([${webkit_package} >= ${WEBKIT_VERSION}],,
+                     [PKG_CHECK_EXISTS([webkit2gtk-4.0 >= ${WEBKIT_VERSION}],
+                                       [webkit_package=webkit2gtk-4.0])])
+
     GP_CHECK_PLUGIN_GTK3_ONLY([webhelper])
     GP_CHECK_PLUGIN_DEPS([WebHelper], [WEBHELPER],
                          [$GP_GTK_PACKAGE >= ${GTK_VERSION}
                           glib-2.0 >= ${GLIB_VERSION}
                           gio-2.0 >= ${GIO_VERSION}
                           gdk-pixbuf-2.0 >= ${GDK_PIXBUF_VERSION}
-                          webkit2gtk-4.0 >= ${WEBKIT_VERSION}
+                          $webkit_package >= ${WEBKIT_VERSION}
                           gthread-2.0])