don't emit -ltbb when no tbb was installed

The Windows branch of tbbLdFlags() named the TBB libraries
unconditionally, dropping the file.exists() guard the previous stub-based
code had. On a build that fell back to tinythread -- configure.R sets
TBB_ENABLED = FALSE only there, when cmake is missing and TBB_LIB is
unset -- RcppParallelLibs() then emitted

    -L<libs> -lRcppParallel -L<lib> -ltbb -ltbbmalloc

for libraries that were never built or shipped, so a downstream package
would fail to link with "cannot find -ltbb". CxxFlags() correctly reports
-DRCPP_PARALLEL_USE_TBB=0 in that configuration, so the package is not
even using TBB.

Name them only when TBB is enabled and the library actually resolves.

Also drop an unverified aside from NEWS: Rtools42 and Rtools45 are
confirmed to ship cmake, Rtools40 was not checked.

Author: kevinushey

NEWS.md

@@ -17,8 +17,8 @@
   against. Building it ourselves gives every platform the same oneTBB and makes
   the ABI a property of RcppParallel. `TBB_LIB` / `TBB_INC` are still honoured
   for anyone supplying their own build. This requires cmake, which Rtools has
-  provided since Rtools42; toolchains without it (e.g. Rtools40) continue to
-  use the tinythread fallback.
+  shipped since Rtools42; toolchains without one continue to use the tinythread
+  fallback.
 
 * As a consequence, `RcppParallel::RcppParallelLibs()` now emits `-ltbb` and
   `-ltbbmalloc` on Windows, in addition to `-lRcppParallel` (which remains

R/tbb.R

@@ -94,16 +94,23 @@ tbbLdFlags <- function() {
    if (is_windows()) {
 
       libsPath <- archSystemFile("libs")
-      tbbPath <- tbbLibraryPath()
-
-      fmt <- "-L%s -lRcppParallel -L%s -l%s -l%s"
-      return(sprintf(
-         fmt,
-         asBuildPath(libsPath),
-         asBuildPath(tbbPath),
-         TBB_NAME,
-         TBB_MALLOC_NAME
-      ))
+      ldFlags <- sprintf("-L%s -lRcppParallel", asBuildPath(libsPath))
+
+      # only name the TBB libraries when there are TBB libraries to name: a
+      # build that fell back to tinythread ships none, and asking the linker
+      # for them would fail the downstream build outright
+      if (TBB_ENABLED && !is.null(tbbLibraryPath("tbb"))) {
+         fmt <- "%s -L%s -l%s -l%s"
+         ldFlags <- sprintf(
+            fmt,
+            ldFlags,
+            asBuildPath(tbbLibraryPath()),
+            TBB_NAME,
+            TBB_MALLOC_NAME
+         )
+      }
+
+      return(ldFlags)
 
    }