test-fuzz-targets: look for clang-6.0 binary as well

Debian sid is still shipped with clang 4.0 by default. This allows me to
run the test without replacing the system clang to clang-6.0.

tests/hghave.py

@@ -709,6 +709,10 b' def has_clang_libfuzzer():'
         return int(mat.group(1)) > 5
     return False
 
+@check("clang-6.0", "clang 6.0 with version suffix (libfuzzer included)")
+def has_clang60():
+    return matchoutput('clang-6.0 --version', b'clang version 6\.')
+
 @check("xdiff", "xdiff algorithm")
 def has_xdiff():
     try:

tests/test-fuzz-targets.t

@@ -1,6 +1,17 b''
-#require clang-libfuzzer test-repo
+#require test-repo
+
   $ cd $TESTDIR/../contrib/fuzz
+
+#if clang-libfuzzer
   $ make -s clean all
+#endif
+#if no-clang-libfuzzer clang-6.0
+  $ make -s clean all CC=clang-6.0 CXX=clang++-6.0
+#endif
+#if no-clang-libfuzzer no-clang-6.0
+  $ exit 80
+#endif
+
 Just run the fuzzers for five seconds each to verify it works at all.
   $ ./bdiff -max_total_time 5
   $ ./xdiff -max_total_time 5