tests: be robust to platforms that support UTF-8 even with LC_ALL=C

Because musl supports UTF-8 with LC_ALL=C, gcc produces:

  input.y: In function ‘yyparse’:

instead of:

  input.y: In function 'yyparse':

Reported by Ferdinand Thiessen.
http://lists.gnu.org/archive/html/bug-bison/2015-02/msg00001.html

* tests/synclines.at (AT_SYNCLINES_COMPILE): Skip syncline tests when
we can't trust error messages issued about a function body.

THANKS

@@ -48,6 +48,7 @@ Enrico Scholz             enrico.scholz@informatik.tu-chemnitz.de
 Eric Blake                ebb9@byu.net
 Evgeny Stambulchik        fnevgeny@plasma-gate.weizmann.ac.il
 Fabrice Bauzac            noon@cote-dazur.com
+Ferdinand Thiessen        ferdinand@fthiessen.de
 Florian Krohm             florian@edamail.fishkill.ibm.com
 Frank Heckenbach          frank@g-n-u.de
 Frans Englich             frans.englich@telia.com