enable tbb by default on musl-based linux (e.g. alpine)

RcppParallel.h gated the default RCPP_PARALLEL_USE_TBB on __gnu_linux__,
a glibc-only predefined macro. On musl-based systems such as Alpine, which
define __linux__ but not __gnu_linux__, the default fell through to 0 --
disabling TBB for any downstream package that does not pass
-DRCPP_PARALLEL_USE_TBB explicitly (i.e. does not use RcppParallel::CxxFlags()
in its Makevars), even when RcppParallel itself was built with a working TBB.

Match Linux via __linux__ instead, which covers both glibc and musl. This is
consistent with the bundled TBB sources, which use __linux__ broadly and only
guard on __GLIBC__ where glibc-specific behavior is required.

Fixes #231

Author: kevinushey

inst/include/RcppParallel.h

@@ -9,8 +9,11 @@
 // (NOTE: Windows TBB is temporarily opt-in for packages for
 // compatibility with CRAN packages not previously configured
 // to link to TBB in Makevars.win)
+// NOTE: match Linux via __linux__ rather than __gnu_linux__; the latter is
+// only defined by glibc toolchains, which left TBB disabled by default on
+// musl-based systems such as Alpine Linux (https://github.com/RcppCore/RcppParallel/issues/231)
 #ifndef RCPP_PARALLEL_USE_TBB
-# if defined(__APPLE__) || defined(__gnu_linux__) || (defined(__sun) && defined(__SVR4) && !defined(__sparc))
+# if defined(__APPLE__) || defined(__linux__) || (defined(__sun) && defined(__SVR4) && !defined(__sparc))
 #  define RCPP_PARALLEL_USE_TBB 1
 # else
 #  define RCPP_PARALLEL_USE_TBB 0