quantum-espresso: correctly cross-compile code for Cray and BG/Q (#17180)

var/spack/repos/builtin/packages/quantum-espresso/package.py

@@ -224,6 +224,19 @@ def install(self, spec, prefix):
         prefix_path = prefix.bin if '@:5.4.0' in spec else prefix
         options = ['-prefix={0}'.format(prefix_path)]
 
+        # This additional flag is needed anytime the target architecture
+        # does not match the host architecture, which results in a binary that
+        # configure cannot execute on the login node. This is how we detect
+        # cross compilation: If the platform is NOT either Linux or Darwin
+        # and the target=backend, that we are in the cross-compile scenario
+        # scenario. This should cover Cray, BG/Q, and other custom platforms.
+        # The other option is to list out all the platform where you would be
+        # cross compiling explicitly.
+        if not (spec.satisfies('platform=linux') or
+                spec.satisfies('platform=darwin')):
+            if spec.satisfies('target=backend'):
+                options.append('--host')
+
         # QE autoconf compiler variables has some limitations:
         # 1. There is no explicit MPICC variable so we must re-purpose
         #    CC for the case of MPI.