Merge branch 'backport-genex-SOURCES-test' into release-3.9

Tests/RunCMake/GeneratorExpression/TARGET_PROPERTY-SOURCES.cmake

@@ -1,5 +1,5 @@
-cmake_policy(SET CMP0070 NEW)
 enable_language(C)
 add_library(foo empty.c empty2.c)
 target_sources(foo PRIVATE empty3.c)
-file(GENERATE OUTPUT foo.txt CONTENT "$<TARGET_PROPERTY:foo,SOURCES>")
+file(GENERATE OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/foo.txt
+     CONTENT "$<TARGET_PROPERTY:foo,SOURCES>")