rPackages: Support building in parallel, do so by default

Some rPackages (e.g. duckdb) take a *very* long time to build without
any parallelization.

pkgs/development/r-modules/generic-builder.nix

@@ -10,8 +10,11 @@ stdenv.mkDerivation ({
   env.NIX_CFLAGS_COMPILE =
     lib.optionalString stdenv.isDarwin "-I${lib.getDev libcxx}/include/c++/v1";
 
+  enableParallelBuilding = true;
+
   configurePhase = ''
     runHook preConfigure
+    export MAKEFLAGS+="''${enableParallelBuilding:+-j$NIX_BUILD_CORES}"
     export R_LIBS_SITE="$R_LIBS_SITE''${R_LIBS_SITE:+:}$out/library"
     runHook postConfigure
   '';