CMake: Make the env variable precedence by default in set_with_default (

#11070)

Summary:
After the modification in #11040, if the `var_name` is already defined, the `set_with_default` method will not override it with the env variable. However, before that, the value from the env will be prioritized for defining `var_name`. The design goal of the `resolve_dependency_url` method is to allow the overriding of values defined in `VELOX_xxx_SOURCE_URL` with env variables, which enables specifying `VELOX_xxx_SOURCE_URL` to a local file to reduce dependency download time and achieve offline dependency compilation.

Pull Request resolved: #11070

Reviewed By: DanielHunte

Differential Revision: D63344157

Pulled By: Yuhta

fbshipit-source-id: 3740d3a77f12f096f7c7c9d5460b005b3a9815e0

CMake/ResolveDependency.cmake

@@ -94,19 +94,16 @@ macro(set_source dependency_name)
     STATUS "Setting ${dependency_name} source to ${${dependency_name}_SOURCE}")
 endmacro()
 
-# If the var_name is not defined then set var_name to the value of
-# $ENV{envvar_name} if it is defined. If neither is defined then set var_name to
-# ${DEFAULT}. If called from within a nested scope the variable will not
-# propagate into outer scopes automatically! Use PARENT_SCOPE.
+# Set var_name to the value of $ENV{envvar_name} if ENV is defined. If neither
+# ENV or var_name is defined then set var_name to ${DEFAULT}. If called from
+# within a nested scope the variable will not propagate into outer scopes
+# automatically! Use PARENT_SCOPE.
 function(set_with_default var_name envvar_name default)
-  if(DEFINED ${var_name})
-    return()
-  endif()
   if(DEFINED ENV{${envvar_name}})
     set(${var_name}
         $ENV{${envvar_name}}
         PARENT_SCOPE)
-  else()
+  elseif(NOT DEFINED ${var_name})
     set(${var_name}
         ${default}
         PARENT_SCOPE)
@@ -142,6 +139,8 @@ macro(resolve_dependency_url dependency_name)
   set_with_default(
     VELOX_${dependency_name}_SOURCE_URL VELOX_${dependency_name}_URL
     ${VELOX_${dependency_name}_SOURCE_URL})
+  message(VERBOSE "Set VELOX_${dependency_name}_SOURCE_URL to "
+          "${VELOX_${dependency_name}_SOURCE_URL}")
   if(DEFINED ENV{VELOX_${dependency_name}_URL})
     set_with_default(VELOX_${dependency_name}_BUILD_SHA256_CHECKSUM
                      VELOX_${dependency_name}_SHA256 "")