load tbb stub library on windows again

RcppParallel 6.0.0 (via #241) stopped loading any TBB library in .onLoad
on Windows, on the grounds that TBB is statically linked there. However,
we still ship a stub tbb.dll (see src/install.libs.R) for packages that
link with '-ltbb', e.g. via StanHeaders' LdFlags(). Those packages record
a load-time dependency on 'tbb.dll' in their DLL's import table, and the
Windows loader can only resolve it against an already-loaded module --
RcppParallel's lib/x64 directory is not on the DLL search path.

As a result, with 6.0.0 on Windows, Stan-based packages fail to load with:

  LoadLibrary failure:  The specified module could not be found.

as seen with e.g. WSPsignal on r-universe:
https://github.com/r-universe/cran/actions/runs/30028540221/job/89334243681

Restore the preload, resolving the stub directly (tbbLibraryPath() now
reports static library names on Windows, so it cannot be used here).

Author: kevinushey

NEWS.md

@@ -1,5 +1,12 @@
 # RcppParallel (development version)
 
+* On Windows, RcppParallel once again loads its compatibility stub library
+  (`tbb.dll`) when the package is loaded. Packages linking with `-ltbb`
+  (e.g. via StanHeaders) record a load-time dependency on `tbb.dll`, which
+  can only be resolved if RcppParallel has already loaded it; with
+  RcppParallel 6.0.0, such packages would fail to load with "LoadLibrary
+  failure: The specified module could not be found".
+
 * When building the bundled copy of oneTBB, RcppParallel no longer searches
   for hwloc, and so no longer tries to build the optional 'tbbbind' library.
   This fixes build failures on machines where a static hwloc library is

R/zzz.R

@@ -11,10 +11,23 @@
 .tbbMallocProxyDllInfo <- NULL
 
 loadTbbLibrary <- function(name) {
-   # TBB is statically linked on Windows
+
+   # On Windows, TBB is statically linked into RcppParallel.dll, but we
+   # still ship a stub tbb.dll for compatibility with packages linking via
+   # '-ltbb' (e.g. through StanHeaders). Such packages record a load-time
+   # dependency on 'tbb.dll', which the Windows loader can only resolve if
+   # we've already loaded it -- the library directory itself is not on the
+   # DLL search path.
    if (is_windows()) {
-      return(NULL)
+
+      path <- file.path(tbbRoot(), paste0(name, ".dll"))
+      if (!file.exists(path))
+         return(NULL)
+
+      return(dyn.load(path, local = FALSE, now = TRUE))
+
    }
+
    path <- tbbLibraryPath(name)
    if (is.null(path))
       return(NULL)