Use configure depends to trigger reconfiguration when LLVMBuild files change

[danielframpton]

Summary

The existing logic has a workaround where configure_file is used to write a single dummy file output many times.

CMake has a feature to more directly add the dependency and avoid the dummy file (it is available in the minimum version specified).

[theraven]

Looks good to me. I presume we didn't use this because of CMake version dependencies. Was this feature available in our current minimum CMake version?

[danielframpton]

In D80218#2044483, @theraven wrote:

Looks good to me. I presume we didn't use this because of CMake version dependencies. Was this feature available in our current minimum CMake version?

It was added in CMake 3.0.0, and we currently require 3.4.3.

The code I am changing here dates from 2011.

[theraven]

That makes sense. Do you have commit access yet? If not, I'll merge this tomorrow.

[theraven]

This seems to be failing on the Windows pre-merge test run. Unfortunately, I don't have access to those logs and so I can't merge this until this issue is fixed: https://github.com/google/llvm-premerge-checks/issues/187

llvm/utils/llvm-build/llvmbuild/main.py

 #===------------------------------------------------------------------------===#
 
 """ % header_string)
 
         # Write the dependency information in the best way we can.
         f.write("""
 # LLVMBuild CMake fragment dependencies.
 #
-# CMake has no builtin way to declare that the configuration depends on
-# a particular file. However, a side effect of configure_file is to add
-# said input file to CMake's internal dependency list. So, we use that
-# and a dummy output file to communicate the dependency information to
-# CMake.
-#
-# FIXME: File a CMake RFE to get a properly supported version of this
-# feature.
 """)
         for dep in dependencies:
             f.write("""\
-configure_file(\"%s\"
-               ${CMAKE_CURRENT_BINARY_DIR}/DummyConfigureOutput)\n""" % (
+set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS \"%s\")\n""" % (
                 cmake_quote_path(dep),))
 
         # Write the properties we use to encode the required library dependency
         # information in a form CMake can easily use directly.
         f.write("""
 # Explicit library dependency information.
 #
 # The following property assignments effectively create a map from component