fix installation on windows without an rtools tbb

On Windows systems whose Rtools does not provide TBB (R < 4.2.0), two
code paths broke the install:

- configure required cmake whenever TBB_LIB was empty, but the bundled
  TBB is never built on Windows; skip the cmake requirement there

- the tbb stub library was built unconditionally on Windows, but
  tbb-compat.cpp references tbb::detail::r1::observe, which nothing
  provides when TBB is disabled, so the link (and thus the install)
  failed; the stub is also useless in that configuration, as
  RcppParallel.dll exports no TBB symbols for old binaries to bind

Author: kevinushey

NEWS.md

@@ -1,5 +1,9 @@
 # RcppParallel (development version)
 
+* Fixed installation on Windows systems whose Rtools does not provide TBB
+  (R < 4.2.0): configure no longer requires cmake there, and the tbb stub
+  library is no longer built when the TBB backend is disabled.
+
 * RcppParallel now reports which TBB headers and libraries are installed
   with the package, and from where, during package installation. In
   addition, setting the `VERBOSE` environment variable to a value other

src/install.libs.R

@@ -76,8 +76,11 @@
    }
 
    # on Windows, we create a stub library that links to us so that
-   # older binaries (like rstan) can still load
-   if (.Platform$OS.type == "windows") {
+   # older binaries (like rstan) can still load. this is only relevant
+   # when TBB is available: the stub cannot link without it, and old
+   # TBB-using binaries could not run against a TBB-less RcppParallel
+   # regardless
+   if (.Platform$OS.type == "windows" && TBB_ENABLED) {
       tbbDll <- file.path(tbbDest, "tbb.dll")
       if (file.exists(tbbDll)) {
          writeLines("** tbb.dll already exists; skipping tbb stub library")

tools/config/configure.R

@@ -267,8 +267,10 @@ if (.Platform$OS.type == "windows") {
 define(PKG_LIBS = paste(pkgLibs, collapse = " "))
 
 # if we're going to build tbb from sources, check for cmake
+# (not required on Windows, where the bundled TBB is never built;
+# without an Rtools TBB, the tinythread fallback is used instead)
 define(CMAKE = "")
-if (is.na(tbbLib)) {
+if (is.na(tbbLib) && .Platform$OS.type != "windows") {
 
    cmake <- local({