Revert "Do not enable builtin_veccore if fail-on-missing=ON"

This reverts commit 901f8d96.

cmake/modules/SearchInstalledSoftware.cmake

@@ -1350,21 +1350,16 @@ elseif(veccore)
   if(vc)
     set(VecCore_COMPONENTS Vc)
   endif()
-  if(fail-on-missing)
-    find_package(VecCore 0.4.2 CONFIG QUIET REQUIRED COMPONENTS ${VecCore_COMPONENTS})
+  find_package(VecCore 0.4.2 CONFIG QUIET COMPONENTS ${VecCore_COMPONENTS})
+  if(NOT VecCore_FOUND)
+    message(STATUS "VecCore not found, switching on 'builtin_veccore' option.")
+    set(builtin_veccore ON CACHE BOOL "" FORCE)
   else()
-    find_package(VecCore 0.4.2 CONFIG QUIET COMPONENTS ${VecCore_COMPONENTS})
-    if(NOT VecCore_FOUND)
-      message(STATUS "VecCore not found, switching on 'builtin_veccore' option.")
-      set(builtin_veccore ON CACHE BOOL "" FORCE)
-    endif()
-  endif()
-  if(VecCore_FOUND)
     set_property(DIRECTORY APPEND PROPERTY INCLUDE_DIRECTORIES ${VecCore_INCLUDE_DIRS})
   endif()
 endif()
 
-if(builtin_veccore)
+if(veccore AND NOT VecCore_FOUND)
   set(VecCore_VERSION "0.4.2")
   set(VecCore_PROJECT "VecCore-${VecCore_VERSION}")
   set(VecCore_SRC_URI "${lcgpackages}/${VecCore_PROJECT}.tar.gz")