turbine: Better auto-detection of R location (#11711)

If R is provided by the system with a module (i.e. registered in
packages.yaml with a "module" entry) and has no discernible prefix,
this allows turbine to still make use of R.

var/spack/repos/builtin/packages/turbine/package.py

@@ -6,6 +6,8 @@
 
 from spack import *
 
+import os
+
 
 class Turbine(AutotoolsPackage):
     """Turbine: The Swift/T runtime"""
@@ -71,6 +73,13 @@ def configure_args(self):
             args.append('--with-python-exe={0}'.format(
                         self.spec['python'].command.path))
         if '+r' in self.spec:
-            args.append('--with-r={0}/rlib/R'.format(
-                        self.spec['r'].prefix))
+            r_location = '{0}/rlib/R'.format(self.spec['r'].prefix)
+            if not os.path.exists(r_location):
+                rscript = which('Rscript')
+                if rscript is not None:
+                    r_location = rscript('-e', 'cat(R.home())', output=str)
+                else:
+                    msg = 'Could not locate Rscript on your PATH!'
+                    raise RuntimeError(msg)
+            args.append('--with-r={0}'.format(r_location))
         return args