build: fix macOS build issues blocking app compilation

- engine: liboakengine.dylib intentionally leaves olive::k_app_version
  to the final executable (olive-version-obj); whitelist that symbol
  with -U on Apple, where undefined symbols in dylibs are an error
  (ELF allows them by default).
- app/common/configwrapper.h: only declare the int64_t/uint64_t
  conversion/assignment overloads on Linux LP64, where they differ
  from qint64/quint64; elsewhere both are long long and the extra
  overloads are redeclarations.

app/common/configwrapper.h

@@ -58,6 +58,10 @@ public:
 	{
 		return static_cast<quint64>(oakengine_config_get_int(key_utf8(), 0));
 	}
+	// int64_t/uint64_t overloads only exist where they differ from
+	// qint64/quint64 (Linux LP64: int64_t is long; on macOS/Windows both are
+	// long long, where declaring them would be a redeclaration).
+#if defined(__linux__)
 	operator int64_t() const
 	{
 		return oakengine_config_get_int(key_utf8(), 0);
@@ -66,6 +70,7 @@ public:
 	{
 		return static_cast<uint64_t>(oakengine_config_get_int(key_utf8(), 0));
 	}
+#endif
 	operator QString() const
 	{
 		char buf[1024];
@@ -103,6 +108,7 @@ public:
 		oakengine_config_set_int(key_utf8(), static_cast<int64_t>(v));
 		return *this;
 	}
+#if defined(__linux__)
 	OakConfigValue &operator=(int64_t v)
 	{
 		oakengine_config_set_int(key_utf8(), v);
@@ -113,6 +119,7 @@ public:
 		oakengine_config_set_int(key_utf8(), static_cast<int64_t>(v));
 		return *this;
 	}
+#endif
 	OakConfigValue &operator=(const QString &v)
 	{
 		const QByteArray utf8 = v.toUtf8();