Xcode: Restore CMAKE_XCODE_GENERATE_SCHEME for custom targets

The target property introduced by commit 413b71485a (Xcode: Create Xcode
schemes per target, 2019-03-11, v3.15.0-rc1~347^2) was accidentally not
initialized by `CMAKE_XCODE_GENERATE_SCHEME` for custom targets.  Fix it
and update the test.

Fixes: #19759

Source/cmTarget.cxx

@@ -364,7 +364,6 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
     initPropValue("UNITY_BUILD_BATCH_SIZE", "8");
 #ifdef __APPLE__
     if (this->GetGlobalGenerator()->IsXcode()) {
-      initProp("XCODE_GENERATE_SCHEME");
       initProp("XCODE_SCHEME_ADDRESS_SANITIZER");
       initProp("XCODE_SCHEME_ADDRESS_SANITIZER_USE_AFTER_RETURN");
       initProp("XCODE_SCHEME_DEBUG_DOCUMENT_VERSIONING");
@@ -385,6 +384,12 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
 #endif
   }
 
+  if (this->GetType() != cmStateEnums::INTERFACE_LIBRARY) {
+    if (this->GetGlobalGenerator()->IsXcode()) {
+      initProp("XCODE_GENERATE_SCHEME");
+    }
+  }
+
   // Setup per-configuration property default values.
   if (this->GetType() != cmStateEnums::UTILITY) {
     static const auto configProps = {

Tests/RunCMake/XcodeProject/XcodeSchemaProperty-check.cmake

@@ -7,6 +7,13 @@ function(check_property property matcher)
   endif()
 endfunction()
 
+function(expect_schema target)
+  set(schema "${RunCMake_TEST_BINARY_DIR}/XcodeSchemaProperty.xcodeproj/xcshareddata/xcschemes/${target}.xcscheme")
+  if(NOT EXISTS ${schema})
+    message(SEND_ERROR "Missing schema for target ${target}")
+  endif()
+endfunction()
+
 function(expect_no_schema target)
   set(schema "${RunCMake_TEST_BINARY_DIR}/XcodeSchemaProperty.xcodeproj/xcshareddata/xcschemes/${target}.xcscheme")
   if(EXISTS ${schema})
@@ -40,3 +47,6 @@ check_property("ENVIRONMENT" [=[key="BAR"]=])
 check_property("ENVIRONMENT" [=[value="bar"]=])
 
 expect_no_schema("NoSchema")
+
+expect_schema("CustomTarget")
+expect_schema("ALL_BUILD")

Tests/RunCMake/XcodeProject/XcodeSchemaProperty.cmake

@@ -38,3 +38,5 @@ create_scheme_for_property(ENVIRONMENT "FOO=foo;BAR=bar")
 
 add_executable(NoSchema main.cpp)
 set_target_properties(NoSchema PROPERTIES XCODE_GENERATE_SCHEME OFF)
+
+add_custom_target(CustomTarget)