Remove the pipe2 symbol from libc.so, otherwise meson will pick it up.

This is a balance.  We decide not to strip all the O_CLOEXEC, otherwise
gtk+ will need to be patched.
Index: glibc-2.19/sysdeps/unix/make-syscalls.sh
===================================================================
--- glibc-2.19.orig/sysdeps/unix/make-syscalls.sh
+++ glibc-2.19/sysdeps/unix/make-syscalls.sh
@@ -131,6 +131,7 @@ emit_weak_aliases()
 	echo "	 echo 'libc_hidden_def ($name)'; \\"
 	;;
       *)
+	[ $name = pipe2 ] && continue
 	echo "	 echo 'weak_alias ($strong, $name)'; \\"
 	echo "	 echo 'libc_hidden_weak ($name)'; \\"
 	;;