Tests: Decouple Plugin test from KWSys

KWSys now requires C++11 but we want this test to be able to run as
C++98.  Copy the KWSys DynamicLoader implementation (with original
notice headers and license reference) and update it to work alone.

Tests/Plugin/include/DynamicLoader.hxx

@@ -0,0 +1,49 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.
+   See https://cmake.org/licensing#kwsys for details.  */
+#ifndef DynamicLoader_hxx
+#define DynamicLoader_hxx
+
+#include <string>
+
+#if defined(__hpux)
+#  include <dl.h>
+#elif defined(_WIN32) && !defined(__CYGWIN__)
+#  include <windows.h>
+#elif defined(__APPLE__)
+#  include <AvailabilityMacros.h>
+#  if MAC_OS_X_VERSION_MAX_ALLOWED < 1030
+#    include <mach-o/dyld.h>
+#  endif
+#elif defined(__BEOS__)
+#  include <be/kernel/image.h>
+#endif
+
+class DynamicLoader
+{
+public:
+#if defined(__hpux)
+  typedef shl_t LibraryHandle;
+#elif defined(_WIN32) && !defined(__CYGWIN__)
+  typedef HMODULE LibraryHandle;
+#elif defined(__APPLE__)
+#  if MAC_OS_X_VERSION_MAX_ALLOWED < 1030
+  typedef NSModule LibraryHandle;
+#  else
+  typedef void* LibraryHandle;
+#  endif
+#elif defined(__BEOS__)
+  typedef image_id LibraryHandle;
+#else // POSIX
+  typedef void* LibraryHandle;
+#endif
+
+  typedef void (*SymbolPointer)();
+
+  static LibraryHandle OpenLibrary(const std::string&);
+
+  static int CloseLibrary(LibraryHandle);
+
+  static SymbolPointer GetSymbolAddress(LibraryHandle, const std::string&);
+};
+
+#endif