tests: don't name C++ files *.c

[Akim Demaille]

commit 2c6b7c8b2c4b4cf04f3368e93d3df786445110c0
Author: Akim Demaille <address@hidden>
Date:   Thu Nov 29 07:44:37 2018 +0100

    tests: don't name C++ files *.c
    
    * tests/synclines.at (syncline escapes): Here.
    Otherwise, Clang generates an error and skips the test.

diff --git a/tests/synclines.at b/tests/synclines.at
index 62f2a76b..b66edb8b 100644
--- a/tests/synclines.at
+++ b/tests/synclines.at
@@ -395,11 +395,11 @@ AT_BISON_OPTION_PUSHDEFS([%skeleton "$1"])
 # ICC is unable to handle these file names: it fails to pass them
 # properly to the linker, and is unable to save a file named this way.
 # Don't try with such compilers.
-AT_DATA([\"\\\"\".c],
+AT_DATA([\"\\\"\".AT_LANG_EXT],
 [[int main (void) { return 0; }
 ]])
 
-AT_CHECK([AT_CXX_IF([$CXX $CXXFLAGS], [$CC $CFLAGS]) $CPPFLAGS \"\\\"\".c -o 
\"\\\"\" ||
+AT_CHECK([AT_CXX_IF([$CXX $CXXFLAGS], [$CC $CFLAGS]) $CPPFLAGS 
\"\\\"\".]AT_LANG_EXT[ -o \"\\\"\" ||
           exit 77], [0], [ignore], [ignore])
 
 AT_DATA_GRAMMAR([\"\\\"\".y],