Use configure depends to trigger reconfiguration when LLVMBuild files change

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).

Reviewers: theraven

Reviewed By: theraven

Subscribers: theraven, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80218

utils/llvm-build/llvmbuild/main.py

@@ -563,19 +563,10 @@ subdirectories = %s
         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