add platform check for how to link to the standard math library.

This allows to support QNX.

cmake/EigenTesting.cmake

@@ -40,6 +40,9 @@ macro(ei_add_test_internal testname testname_with_suffix)
     ei_add_target_property(${targetname} COMPILE_FLAGS "${ARGV2}")
   endif(${ARGC} GREATER 2)
 
+  if(EIGEN_STANDARD_LIBRARIES_TO_LINK_TO)
+    target_link_libraries(${targetname} ${EIGEN_STANDARD_LIBRARIES_TO_LINK_TO})
+  endif()
   if(EXTERNAL_LIBS)
     target_link_libraries(${targetname} ${EXTERNAL_LIBS})
   endif()