blas_lapack: add multithreading variant consistent in all implementations. (#5340)

* blas_lapack: add multithreading variant

* elemental: update

* intel-mkl: extend to macOS

* rename multithreading to threads

* intel-mkl: avoid long lines

* intel-mkl: make one install error a conflict

* openblas: fix a minor bug in the test

var/spack/repos/builtin/packages/intel-parallel-studio/package.py

@@ -104,8 +104,12 @@ class IntelParallelStudio(IntelPackage):
             description='Builds shared library')
     variant('ilp64',    default=False,
             description='64 bit integers')
-    variant('openmp',   default=False,
-            description='OpenMP multithreading layer')
+    variant(
+        'threads', default='none',
+        description='Multithreading support',
+        values=('openmp', 'none'),
+        multi=False
+    )
 
     # Components available in all editions
     variant('daal', default=True,
@@ -168,7 +172,7 @@ def blas_libs(self):
 
         omp_libs = LibraryList([])
 
-        if '+openmp' in spec:
+        if spec.satisfies('threads=openmp'):
             if '%intel' in spec:
                 mkl_threading = ['libmkl_intel_thread']
                 omp_threading = ['libiomp5']