webkit: mention build failure with old linuxthread toolchains

I could have created a !dependency on !BR2_PTHREADS_OLD, but that
wouldn't work with external toolchains. So, let's just add a notice in
the help message of WebKit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/webkit/Config.in

@@ -14,6 +14,9 @@ config BR2_PACKAGE_WEBKIT
 	help
 	  WebKit is an open source, standards compliant web browser engine.
 
+	  Note that WebKit does not build with a toolchain using the
+	  old linuxthreads library.
+
 	  http://webkit.org/
 
 comment "webkit requires a toolchain with C++ support and WCHAR enabled"