Opt avx (#7486)

* Promote to Autotools for simplicity; broaden Intel CPU opt targeting from SSE4.2 to AVX2

* make Intel CPU opt targeting same as in ../libxc

* flake8 W291

* use canonical means to pass (ahem) the test phase, h/t @adamjstewart

* revert f25d598 (unrelated merge)

* re-merge

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

@@ -64,11 +64,10 @@ def autoreconf(self, spec, prefix):
     @property
     def optflags(self):
         flags = '-O2'
-
         # Optimizations for the Intel compiler, suggested by CP2K
+        # See ../libxc/package.py for rationale and doc.
         if '%intel' in self.spec:
-            # -xSSE2 will make it usable on old architecture
-            flags += ' -xSSE2 -xAVX -axCORE-AVX2 -ipo'
+            flags += ' -xSSE4.2 -axAVX,CORE-AVX2 -ipo'
 
         return flags