link downstream packages against the tbb stub on windows

tbbLdFlags() returned only '-L<libs> -lRcppParallel' on Windows, leaving
downstream packages to resolve TBB symbols against whichever objects
happened to be pulled out of the static TBB when RcppParallel.dll was
linked. That export surface is incidental rather than declared, and
packages taking all of their PKG_LIBS from RcppParallelLibs() (e.g. rstan)
have no other way to ask for TBB.

Offer the stub library as well, after '-lRcppParallel' so the package's own
exports still take precedence and no dependency on the stub is recorded
unless something actually needs it. RcppParallel 5.1.11 and earlier appended
'-ltbb -ltbbmalloc' here for the same reason.

Author: kevinushey

R/tbb.R

@@ -87,12 +87,25 @@ tbbLdFlags <- function() {
 
    # on Windows, we statically link to oneTBB
    if (is_windows()) {
-      
+
       libPath <- archSystemFile("libs")
 
       ldFlags <- sprintf("-L%s -lRcppParallel", asBuildPath(libPath))
+
+      # also offer the stub library. RcppParallel.dll re-exports whichever TBB
+      # objects happened to be pulled out of the static library when it was
+      # linked, which is an incidental export surface rather than a declared
+      # one; the stub exports the runtime wholesale, so anything missing from
+      # RcppParallel.dll can still be resolved. this comes last deliberately:
+      # the linker satisfies symbols in order, so RcppParallel's own exports
+      # still win, and no dependency on the stub is recorded unless something
+      # actually needs it
+      tbbPath <- archSystemFile("lib")
+      if (file.exists(file.path(tbbPath, "tbb.dll")))
+         ldFlags <- paste(ldFlags, sprintf("-L%s -ltbb", asBuildPath(tbbPath)))
+
       return(ldFlags)
-      
+
    }
 
    # shortcut if TBB_LIB defined