cmake: required v3.7 for some DOWNLOAD options

cmake/CMakeLists.txt

@@ -377,6 +377,9 @@ endif()
 if(PKG_LATTE)
   option(DOWNLOAD_LATTE "Download latte (instead of using the system's one)" OFF)
   if(DOWNLOAD_LATTE)
+    if (CMAKE_VERSION VERSION_LESS "3.7") # due to SOURCE_SUBDIR 
+      message(FATAL_ERROR "For downlading LATTE you need at least cmake-3.7")
+    endif()
     message(STATUS "LATTE not found - we will build our own")
     include(ExternalProject)
     ExternalProject_Add(latte_build
@@ -479,6 +482,9 @@ if(PKG_MSCG)
   find_package(GSL REQUIRED)
   option(DOWNLOAD_MSCG "Download latte (instead of using the system's one)" OFF)
   if(DOWNLOAD_MSCG)
+    if (CMAKE_VERSION VERSION_LESS "3.7") # due to SOURCE_SUBDIR 
+      message(FATAL_ERROR "For downlading LATTE you need at least cmake-3.7")
+    endif()
     include(ExternalProject)
     if(NOT LAPACK_FOUND)
       set(EXTRA_MSCG_OPTS "-DLAPACK_LIBRARIES=${CMAKE_CURRENT_BINARY_DIR}/liblinalg.a")