openblas: add ilp64 option (#5289)

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

@@ -47,6 +47,7 @@ class Openblas(MakefilePackage):
         default=True,
         description='Build shared libraries as well as static libs.'
     )
+    variant('ilp64', default=False, description='64 bit integers')
     variant('openmp', default=False, description="Enable OpenMP support.")
     variant('pic', default=True, description='Build position independent code')
 
@@ -129,6 +130,10 @@ def make_defs(self):
         if '+openmp' in self.spec:
             make_defs += ['USE_OPENMP=1']
 
+        # 64bit ints
+        if '+ilp64' in self.spec:
+            make_defs += ['INTERFACE64=1']
+
         return make_defs
 
     @property