[cmake] fftw3 ON is allowed to turn on builtin_fftw3 as in the rest of features

Author: ferdymercury

cmake/modules/SearchInstalledSoftware.cmake

@@ -915,13 +915,17 @@ if(fftw3)
     message(STATUS "Looking for FFTW3")
     find_package(FFTW)
     if(NOT FFTW_FOUND)
-      if(fail-on-missing)
-        message(SEND_ERROR "FFTW3 libraries not found and they are required (fftw3 option enabled)")
-      else()
-        message(STATUS "FFTW3 not found. Set [environment] variable FFTW_DIR to point to your FFTW3 installation")
-        message(STATUS "                 Alternatively, you can also enable the option 'builtin_fftw3' to build FFTW3 internally'")
-        message(STATUS "                 For the time being switching OFF 'fftw3' option")
-        set(fftw3 OFF CACHE BOOL "Disabled because FFTW3 not found and builtin_fftw3 disabled (${fftw3_description})" FORCE)
+      set(builtin_fftw3 ON CACHE BOOL "Enabled because FFTW3 not found (${fftw3_description})" FORCE)
+      ROOT_CHECK_CONNECTION_AND_DISABLE_OPTION("builtin_fftw3")
+      if(NOT builtin_fftw3)
+        if(fail-on-missing)
+          message(SEND_ERROR "FFTW3 libraries not found and they are required (fftw3 option enabled)")
+        else()
+          message(STATUS "FFTW3 not found. Set [environment] variable FFTW_DIR to point to your FFTW3 installation")
+          message(STATUS "                 Alternatively, you can also enable your Internet connection so that the autoset option 'builtin_fftw3' can build FFTW3 internally'")
+          message(STATUS "                 For the time being switching OFF 'fftw3' option")
+          set(fftw3 OFF CACHE BOOL "Disabled because FFTW3 not found and builtin_fftw3 unavailable (${fftw3_description})" FORCE)
+        endif()
       endif()
     endif()
   endif()