[Patch, build] Unbreak objc build

[Tobias Burnus]

When I tried to bootstrap with --enable-languages=default, which 
includes objc, it failed to build libobj.

As Rhy0lite on IRC correctly guessed, the issue is that the cet.m4 file 
is at a different location. I wonder why no one had the problem before 
in the last years – the libobjc/Makefile.in change was r254904 on Nov 
17, 2017 (!). The config/cet.m4 file itself was also added on 2017-11-17 
in r254868.

Doing some more archeology, the the Makefile.in was changed by the patch 
https://gcc.gnu.org/ml/gcc-patches/2017-10/msg02314.html – and one sees 
that libobjc/aclocal.m4 has /../config/cet.m4 while libobjc/Makefile.in 
uses /../cet.m4.m4. – Actually, that's not the submitted version 
(cet.m4.m4) – as the committed version has 'cet.m4'. Approval was: 
https://gcc.gnu.org/ml/gcc-patches/2017-11/msg00641.html

Build on x86-64-gnu-linux.
OK for the trunk?

Tobias

PS: Assuming the build was indeed broken for everyone since 2 years, 
maybe one should remove 'objc' from the 'default' languages? – Which 
languages is now nowadays supposed to use when bootstrapping? I usually 
have 'c,c++,fortran,lto', 'default' seems to be 'c,c++,fortran,objc' 
(plus 'lto' which is implied by --enable-lto, which is default).

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