hghave: add test for clang 6 and later

clang 6 includes integrated libfuzzer support, which makes fuzzer
stuff a little easier to work with.

Differential Revision: https://phab.mercurial-scm.org/D1873

tests/hghave.py

@@ -700,3 +700,11 b' def has_fuzzywuzzy():'
         return True
     except ImportError:
         return False
+
+@check("clang-libfuzzer", "clang new enough to include libfuzzer")
+def has_clang_libfuzzer():
+    mat = matchoutput('clang --version', 'clang version (\d)')
+    if mat:
+        # libfuzzer is new in clang 6
+        return int(mat.group(1)) > 5
+    return False