Limit instantiation of GncInt128 constructors to integral values.

Instead of using static_assert. This prevents the compiler from even
trying and avoids weird compilation errors when testing types for
instantiating other templates.

libgnucash/engine/gnc-int128.hpp

@@ -90,38 +90,25 @@ enum // Values for m_flags
  */
 /** Default constructor. Makes 0. */
     GncInt128();
-    template <typename T>
+    template <typename T,
+              std::enable_if_t<std::is_integral<T>::value, bool> = true>
     GncInt128(T lower) : GncInt128(INT64_C(0), static_cast<int64_t>(lower))
-    {
-        static_assert (std::is_integral<T>(),
-                       "GncInt128 can be constructed only with "
-                       "integral arguments.");
-    }
+    {}
     GncInt128(uint64_t lower) : GncInt128 {UINT64_C(0), lower} {}
 /** Double-integer constructor template.
  */
-    template <typename T, typename U>
+    template <typename T, typename U,
+              std::enable_if_t<(std::is_integral<T>::value &&
+              std::is_integral<U>::value), bool> = true>
     GncInt128(T upper, U lower, unsigned char flags = '\0') :
         GncInt128 {static_cast<int64_t>(upper),
-                   static_cast<int64_t>(lower), flags}
-    {
-        static_assert (std::is_integral<T>(),
-                       "GncInt128 can be constructed only with "
-                       "integral arguments.");
-        static_assert (std::is_integral<U>(),
-                       "GncInt128 can be constructed only with "
-                       "integral arguments.");
-    }
+                   static_cast<int64_t>(lower), flags} {}
 
     GncInt128 (int64_t upper, int64_t lower, unsigned char flags = '\0');
-    template <typename T>
+    template <typename T,
+              std::enable_if_t<std::is_integral<T>::value, bool> = true>
     GncInt128(T upper, uint64_t lower) :
-        GncInt128 {static_cast<int64_t>(upper), lower}
-        {
-            static_assert (std::is_integral<T>(),
-                           "GncInt128 can be constructed only with "
-                           "integral arguments.");
-        }
+        GncInt128 {static_cast<int64_t>(upper), lower} {}
 
     GncInt128 (int64_t upper, uint64_t lower, unsigned char flags = '\0');
     GncInt128 (uint64_t upper, uint64_t lower, unsigned char flags = '\0');