file: Fix CONFIGURE output relative path

In commit a6fee09484 (file: Add CONFIGURE subcommand, 2020-03-06,
v3.18.0-rc1~584^2) we accidentally treated relative path outputs
with respect to the current working directory.  Treat them with
respect to the current binary directory instead.

Fixes: #20885

Help/command/file.rst

@@ -508,8 +508,7 @@ The arguments are:
 
 ``OUTPUT <output-file>``
   Specify the output file name to generate. A relative path is treated with
-  respect to the value of :variable:`CMAKE_CURRENT_BINARY_DIR`. See policy
-  :policy:`CMP0070`.
+  respect to the value of :variable:`CMAKE_CURRENT_BINARY_DIR`.
   ``<output-file>`` does not support generator expressions.
 
 ``CONTENT <content>``

Source/cmFileCommand.cxx

@@ -2862,7 +2862,8 @@ bool HandleConfigureCommand(std::vector<std::string> const& args,
 
   // Check for generator expressions
   const std::string input = args[4];
-  std::string outputFile = args[2];
+  std::string outputFile = cmSystemTools::CollapseFullPath(
+    args[2], status.GetMakefile().GetCurrentBinaryDirectory());
 
   std::string::size_type pos = input.find_first_of("<>");
   if (pos != std::string::npos) {

Tests/RunCMake/File_Configure/RunCMakeTest.cmake

@@ -7,3 +7,4 @@ run_cmake(DirOutput)
 run_cmake(NewLineStyle-NoArg)
 run_cmake(NewLineStyle-ValidArg)
 run_cmake(NewLineStyle-WrongArg)
+run_cmake(SubDir)

Tests/RunCMake/File_Configure/SubDir-check.cmake

@@ -0,0 +1,4 @@
+set(expected "${RunCMake_TEST_BINARY_DIR}/SubDir/out.txt")
+if(NOT EXISTS "${expected}")
+  set(RunCMake_TEST_FAILED "Expected file not created:\n  ${expected}")
+endif()

Tests/RunCMake/File_Configure/SubDir.cmake

@@ -0,0 +1 @@
+add_subdirectory(SubDir)

Tests/RunCMake/File_Configure/SubDir/CMakeLists.txt

@@ -0,0 +1 @@
+file(CONFIGURE OUTPUT out.txt CONTENT "")