[Issue 2154] Correct error when building with IBM's latest XLC compil…

…er with xlclang++ front-end.

On AIX, the XLC 16.1.0.1 compiler considers the call to `std::abs` ambigious, so it needs help with a static_cast to the type of the template argument.

include/internal/catch_matchers_floating.cpp

@@ -55,7 +55,7 @@ namespace {
             return lhs == rhs;
         }
 
-        auto ulpDiff = std::abs(lc - rc);
+        auto ulpDiff = std::abs(static_cast<FP>(lc - rc));
         return static_cast<uint64_t>(ulpDiff) <= maxUlpDiff;
     }