don't redeclare the legacy observer classes in the tbb stub

The bundled oneTBB carries a local 'provided for backwards compatibility'
block declaring tbb::internal::task_scheduler_observer_v3 and
tbb::interface6::task_scheduler_observer (added in c276b03; not upstream).
tbb-compat.cpp declares them too, which was fine while the stub was only
ever built against the pristine headers Rtools ships -- but building the
bundled oneTBB on Windows makes the two collide:

  tbb-compat.cpp:25:7: error: redefinition of
    'class tbb::internal::task_scheduler_observer_v3'

Have the bundled header advertise what it provides, and skip the stub's own
declarations when it does. Either way the stub still exports the out-of-line
observe(), which is the whole reason it exists.

Also spell that definition __TBB_EXPORTED_METHOD, matching its declaration
here and in the old TBB ABI; the two attributes are both no-ops on the
Windows targets we build for, so this is consistency rather than a fix.

Author: kevinushey

src/tbb-compat/tbb-compat.cpp

@@ -14,6 +14,13 @@
 # define DLL_EXPORT
 #endif
 
+// The old ABI's observer classes, as they stood before oneTBB. The bundled
+// copy of oneTBB carries these too (see task_scheduler_observer.h), so only
+// declare them when compiling against headers that don't -- i.e. the pristine
+// ones Rtools ships. Either way, the out-of-line observe() below is what this
+// stub exists to export.
+#ifndef __TBB_LEGACY_TASK_SCHEDULER_OBSERVER_PROVIDED
+
 namespace tbb {
 
 namespace interface6 {
@@ -130,11 +137,13 @@ class task_scheduler_observer : public internal::task_scheduler_observer_v3 {
 
 } // namespace tbb
 
+#endif /* __TBB_LEGACY_TASK_SCHEDULER_OBSERVER_PROVIDED */
+
 namespace tbb {
 namespace internal {
 
 DLL_EXPORT
-void __TBB_EXPORTED_FUNC task_scheduler_observer_v3::observe( bool enable ) {
+void __TBB_EXPORTED_METHOD task_scheduler_observer_v3::observe( bool enable ) {
    auto* tso = (tbb::detail::d1::task_scheduler_observer*) (this);
    tbb::detail::r1::observe(*tso, enable);
 }

src/tbb/include/oneapi/tbb/task_scheduler_observer.h

@@ -116,7 +116,13 @@ inline namespace v1 {
 } // namespace tbb
 
 
-// Provided for backwards compatibility.
+// Provided for backwards compatibility. This is a local addition, not part of
+// upstream oneTBB, so the tbb.dll stub built by src/install.libs.R must be
+// able to tell whether the TBB headers in use supply these declarations (the
+// bundled copy does) or whether it has to declare them itself (as with the
+// pristine headers shipped by Rtools).
+#define __TBB_LEGACY_TASK_SCHEDULER_OBSERVER_PROVIDED 1
+
 namespace tbb {
 namespace interface6 {
 class task_scheduler_observer;