check whether BACKEND is either MTL, EIGEN, or ISTL

cmake/modules/AmdisMacros.cmake

@@ -4,7 +4,7 @@ include(AddAmdisExecutable)
 set(BACKEND "ISTL" CACHE STRING "LinearAlgebra backend. One of MTL, EIGEN, ISTL")
 set_property(CACHE BACKEND PROPERTY STRINGS "MTL" "EIGEN" "ISTL")
 
-if (BACKEND STREQUAL "MTL")
+if (BACKEND STREQUAL "MTL" OR BACKEND STREQUAL "MTL4")
   find_package(MTL REQUIRED
     PATHS /usr/local/lib/mtl4 /opt/sources/mtl4 /opt/development/mtl4)
 
@@ -28,7 +28,7 @@ if (BACKEND STREQUAL "MTL")
       COMPILE_DEFINITIONS ${MTL_COMPILE_DEFINITIONS}
       INCLUDE_DIRS ${MTL_INCLUDE_DIRS})
   endif (MTL_FOUND)
-elseif (BACKEND STREQUAL "EIGEN")
+elseif (BACKEND STREQUAL "EIGEN" OR BACKEND STREQUAL "EIGEN3")
   find_package(Eigen3 REQUIRED 3.3.5)
 
   set(HAVE_EIGEN EIGEN_FOUND)
@@ -39,6 +39,10 @@ elseif (BACKEND STREQUAL "EIGEN")
       COMPILE_DEFINITIONS ${EIGEN3_DEFINITIONS}
       INCLUDE_DIRS ${EIGEN3_INCLUDE_DIRS})
   endif (EIGEN3_FOUND)
-elseif (NOT dune-istl_FOUND)
-  message(FATAL "Need dune-istl, MTL, or Eigen3 as linear algebra backend. Change flag BACKEND!")
+elseif (BACKEND STREQUAL "ISTL")
+  if (NOT dune-istl_FOUND)
+    message(FATAL_ERROR "Need dune-istl, MTL, or Eigen3 as linear algebra backend. Change flag BACKEND!")
+  endif ()
+else ()
+  message(FATAL_ERROR "BACKEND must be one of MTL, EIGEN, ISTL")
 endif ()