[cmake] link to arpack++ if found

The arpack++ package from Debian comes with a shared library that we
have to link to. Otherwise the linker will complain about missing
symbols when we try to build arpackpptest.
As there seem to be other versions of arpack++ that are header-only, we
only link to the shared library if it was found.

cmake/modules/FindARPACKPP.cmake

@@ -98,6 +98,15 @@ find_path(ARPACKPP_INCLUDE_DIR
   PATH_SUFFIXES "include" "include/arpack++"
 )
 
+# The arpack++ package in Debian also includes a shared library that we have
+# to link to. Other versions of arpack++ are header-only.
+# Thus we will later use the arpack++ shared library if found and just ignore
+# it if it was not found.
+find_library(ARPACKPP_LIBRARY
+  NAMES "arpack++"
+  PATH_SUFFIXES "lib" "lib32" "lib64"
+)
+
 # check header usability
 include(CMakePushCheckState)
 cmake_push_check_state()
@@ -109,7 +118,14 @@ cmake_push_check_state()
 # correctly in the CMake files."
 if(ARPACKPP_INCLUDE_DIR)
   set(CMAKE_REQUIRED_INCLUDES ${CMAKE_REQUIRED_INCLUDES} ${ARPACKPP_INCLUDE_DIR})
-  set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${ARPACK_LIBRARIES}) #${BLAS_LIBRARIES} ${LAPACK_LIBRARIES} ${UMFPACK_LIBRARIES} ${SUPERLU_LIBRARIES}
+  if(ARPACKPP_LIBRARY)
+    set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES}
+                                 ${ARPACK_LIBRARIES}
+                                 ${ARPACKPP_LIBRARY})
+  else(ARPACKPP_LIBRARY)
+    set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES}
+                                 ${ARPACK_LIBRARIES})
+  endif(ARPACKPP_LIBRARY)
 endif(ARPACKPP_INCLUDE_DIR)
 
 # end of header usability check
@@ -129,7 +145,11 @@ mark_as_advanced(ARPACKPP_INCLUDE_DIR)
 # if headers are found, store results
 if(ARPACKPP_FOUND)
   set(ARPACKPP_INCLUDE_DIRS ${ARPACKPP_INCLUDE_DIR})
-  set(ARPACKPP_LIBRARIES ${ARPACK_LIBRARIES}) #${BLAS_LIBRARIES} ${LAPACK_LIBRARIES} ${UMFPACK_LIBRARIES} ${SUPERLU_LIBRARIES}
+  if(ARPACKPP_LIBRARY)
+    set(ARPACKPP_LIBRARIES ${ARPACKPP_LIBRARY} ${ARPACK_LIBRARIES})
+  else(ARPACKPP_LIBRARY)
+    set(ARPACKPP_LIBRARIES ${ARPACK_LIBRARIES})
+  endif(ARPACKPP_LIBRARY)
   # log result
   file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeOutput.log
     "Determing location of ARPACK++ succeded:\n"