Check for heffte host backends at configure

CMakeLists.txt

@@ -99,6 +99,12 @@ set_package_properties(HYPRE PROPERTIES TYPE OPTIONAL PURPOSE "Used for structur
 # find heffte
 Cabana_add_dependency( PACKAGE Heffte VERSION 2.3.0 )
 set_package_properties(Heffte PROPERTIES TYPE OPTIONAL PURPOSE "Used for fft calculations")
+if(HEFFTE_FOUND)
+  # ensure at least one host backend is enabled
+  if(NOT Heffte_ENABLE_FFTW AND NOT Heffte_ENABLE_MKL)
+    message(FATAL_ERROR "Cabana heFFTe support requires at least one host backend (FFTW or MKL).")
+  endif()
+endif()
 
 # find Silo
 Cabana_add_dependency( PACKAGE SILO )