Fix MacOS 10.11 (clang-8.0.0) builds

gcc, and recent clang-10 can construct new objects
using references as arguments.

However clang-8 (and MSVC?) do not:
   "error: no matching function for call to 'operator new'"

The compiler apparently does not expand the template
  class A  <-> `A*`  vs. `A const&`
for different cases.

1 files changed, 7 insertions, 5 deletions

diff --git a/libs/pbd/pbd/stack_allocator.h b/libs/pbd/pbd/stack_allocator.h
index d719b5c5f3..9eed081ec0 100644
--- a/libs/pbd/pbd/stack_allocator.h
+++ b/libs/pbd/pbd/stack_allocator.h
@@ -128,17 +128,19 @@ public:
 		new (p) U ();
 	}
 
+#if __cplusplus > 201103L || defined __clang__
 	template <class U, class A>
-	void construct (U* const p, A& a)
+	void construct (U* const p, A* const a)
 	{
 		new (p) U (a);
 	}
-
-	template <class U, class A, class B>
-	void construct (U* const p, A& a, B& b)
+#else
+	template <class U, class A>
+	void construct (U* const p, A const& a)
 	{
-		new (p) U (a, b);
+		new (p) U (a);
 	}
+#endif
 
 private:
 	StackAllocator& operator= (const StackAllocator&);