Make the C tests fail with -Wstrict-prototypes

Set CCFLAGS to -Werro -Wstrict-prototypes so that gcc fails in case a
function is declared or defined without specifying the argument types.
This serves as a regression test for pull request #185.

testing/src/ParameterizedTests.hs

@@ -152,6 +152,7 @@ parameters =
          , tpBnfcOptions = ["--ocaml", "-m"] }
   -- C
   , cBase { tpName = "C"
+          , tpBuild = run_ "make" ["CCFLAGS=-Wstrict-prototypes -Werror"]
           , tpBnfcOptions = ["--c", "-m"] }
   -- C++
   , cBase { tpName = "C++"