provide a stacktrace function callable directly from C (libpbd_c_stacktrace())

libs/pbd/pbd/stacktrace.h

@@ -38,6 +38,8 @@
 #include "pbd/libpbd_visibility.h"
 
 
+extern "C" { void libpbd_c_stacktrace (int levels); }
+
 namespace PBD {
 
 	LIBPBD_API void stacktrace (std::ostream& out, int levels = 0, size_t start_level = 0);

libs/pbd/stacktrace.cc

@@ -131,3 +131,13 @@ PBD::stacktrace (std::ostream& out, int, size_t)
 }
 
 #endif
+
+extern "C" {
+
+void
+libpbd_c_stacktrace (int levels)
+{
+	PBD::stacktrace (std::cerr, levels);
+}
+
+}