drop StanHeaders:::LdFlags() TBB linking, incompatible with RcppParallel 6.0.0 on Windows

With rstan now loading correctly (previous commit), Windows CI got
further and hit a second, distinct failure: our own compiled
EpiStrainDynamics.dll failed to load with the identical "LoadLibrary
failure" signature, this time for tbb.dll.

StanHeaders:::LdFlags() unconditionally returns
"-L<TBB_LIB> -Wl,-rpath,<TBB_LIB> -ltbb -ltbbmalloc", assuming
RcppParallel ships a dynamic tbb.dll/tbbmalloc.dll. RcppParallel 6.0.0
(installed on this runner) links TBB statically on Windows instead,
and -Wl,-rpath is a no-op on Windows in any case (it's an ELF/Unix
linker mechanism).

rstan's own from-source Windows build, using the same RcppParallel
6.0.0, links only "-lRcppParallel" with no separate TBB flags and
loads fine -- confirming RcppParallel::RcppParallelLibs() alone
already provides whatever the installed RcppParallel version actually
needs. Drop the StanHeaders:::LdFlags() call from PKG_LIBS in
Makevars.win to match.

Author: smwindecker

NEWS.md

@@ -13,17 +13,27 @@
   explicitly importing `%||%` from `rlang` instead of relying on base R's
   version, which only exists from R 4.4.0 (related to #43).
 * Fixed intermittent R CMD check failures on Windows CI. The reported
-  error ("Build process failed") was misleading: all six bundled Stan
-  models were actually compiling and linking successfully, but the
-  package then failed to lazy-load because `rstan`'s own precompiled
-  Windows binary DLL failed to load ("LoadLibrary failure: The
-  specified module could not be found"), most likely an ABI mismatch
-  between the RSPM binary and the runner's Rtools version. Fixed by
-  reinstalling `rstan` from source on the Windows CI job specifically.
-  Also added `src/Makevars.win` (lowering optimisation from `-O2` to
-  `-O1` and suppressing compiler warnings) and enlarged the Windows
-  runner's pagefile; these weren't the actual fix, but remain in place
-  as they reduce build time/noise and are otherwise harmless.
+  error ("Build process failed") was misleading and had two distinct,
+  unrelated causes hiding behind it, both only visible once compiler
+  warning noise was suppressed (see below):
+  - `rstan`'s own precompiled Windows binary DLL failed to load
+    ("LoadLibrary failure"), most likely an ABI mismatch between the
+    RSPM binary and the runner's Rtools version. Fixed by reinstalling
+    `rstan` from source on the Windows CI job specifically.
+  - With that resolved, the package's own compiled DLL then failed to
+    load the same way, because `StanHeaders:::LdFlags()` unconditionally
+    links against a *dynamic* `tbb`/`tbbmalloc`, which `RcppParallel`
+    (from version 6.0.0) no longer reliably provides on Windows (having
+    moved to static TBB linking), and `-Wl,-rpath` has no effect on
+    Windows regardless. Fixed by dropping `StanHeaders:::LdFlags()` from
+    `src/Makevars.win`'s `PKG_LIBS` and relying solely on
+    `RcppParallel::RcppParallelLibs()`, matching what `rstan`'s own
+    Windows build does.
+  Also lowered optimisation from `-O2` to `-O1`, suppressed compiler
+  warnings, and enlarged the Windows runner's pagefile in
+  `src/Makevars.win`/CI config; these weren't the actual fix for either
+  issue above, but remain in place as they reduce build time/noise and
+  are otherwise harmless.
 
 ## Minor improvements
 

src/Makevars.win

@@ -3,7 +3,21 @@ STANHEADERS_SRC = $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "message()" -e
 STANC_FLAGS = $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "cat(ifelse(utils::packageVersion('rstan') >= '2.26', '-DUSE_STANC3',''))")
 PKG_CPPFLAGS = -I"../inst/include" -I"$(STANHEADERS_SRC)" -DBOOST_DISABLE_ASSERTS -DEIGEN_NO_DEBUG -DBOOST_MATH_OVERFLOW_ERROR_POLICY=errno_on_error $(STANC_FLAGS) -D_HAS_AUTO_PTR_ETC=0
 PKG_CXXFLAGS = $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "RcppParallel::CxxFlags()") $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "StanHeaders:::CxxFlags()")
-PKG_LIBS = $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "RcppParallel::RcppParallelLibs()") $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "StanHeaders:::LdFlags()")
+
+# Deliberately omits StanHeaders:::LdFlags() (unlike the Unix Makevars):
+# that function unconditionally emits "-L<TBB_LIB> -Wl,-rpath,<TBB_LIB>
+# -ltbb -ltbbmalloc", which assumes RcppParallel ships a *dynamic*
+# tbb.dll/tbbmalloc.dll -- true for older RcppParallel releases, but
+# RcppParallel >= 6.0.0 links TBB statically on Windows and no longer
+# reliably provides those at the expected path, and "-Wl,-rpath" has no
+# effect on Windows regardless (it's an ELF/Unix mechanism). The result
+# was our own compiled EpiStrainDynamics.dll failing to load at runtime
+# with "LoadLibrary failure: The specified module could not be found"
+# for tbb.dll. RcppParallel::RcppParallelLibs() alone already provides
+# whatever linking its own installed version actually needs -- this is
+# confirmed by rstan's own from-source Windows build linking only
+# -lRcppParallel with no separate TBB flags at all.
+PKG_LIBS = $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript" -e "RcppParallel::RcppParallelLibs()")
 
 CXX_STD = CXX17