use configure-detected TBB_LIB during install.libs (#254)

At the install-libs phase, TBB_LIB was read only from the environment,
which is typically unset there -- configure communicates it via make
variables, not the environment. On Windows, where configure detects the
Rtools copy of TBB, this meant .install.libs() wrongly took the
bundled-TBB branch, shipping any stale artifacts present in
tbb/build/lib_release (e.g. TBB DLLs built by RcppParallel 5.x in a
long-lived working tree, masking the tbb stub library). On Linux and
macOS, system-TBB builds silently shipped no libraries at all.

Fall back to the configure-detected value from tbb-autodetected.R when
the environment variable is unset.

Author: kevinushey

src/install.libs.R

@@ -319,6 +319,15 @@ if (identical(args, "build")) {
       useBundledTbb()
    }
 } else {
+
+   # prefer the configure-detected TBB_LIB when the environment variable
+   # is unset; otherwise, e.g. on Windows (where configure detects the
+   # Rtools copy of TBB), we'd wrongly take the bundled-TBB branch below,
+   # and ship any stale artifacts present in tbb/build/lib_release
    source("../R/tbb-autodetected.R")
+   if (!nzchar(tbbLib))
+      tbbLib <- TBB_LIB
+
    .install.libs(tbbLib)
+
 }