Add a switch to not include git commit hash in the built artifacts

Author: hlopko

CMakeLists.txt

@@ -54,6 +54,10 @@ include(CheckSymbolExists)
 # This is primarily to support building smaller or faster project files.
 #
 
+option(SWIFT_APPEND_VC_REV
+  "Embed the version control system revision in Swift"
+  TRUE)
+
 option(SWIFT_INCLUDE_TOOLS
     "Generate build targets for swift tools"
     TRUE)

lib/Basic/CMakeLists.txt

@@ -12,7 +12,11 @@ else()
 endif()
 
 function(generate_revision_inc revision_inc_var name dir)
-  find_first_existing_vc_file("${dir}" ${name}_vc)
+  if(SWIFT_APPEND_VC_REV)
+    # generate_vcs_version_script generates header with only `undef`s
+    # inside when source directory doesn't exist.
+    find_first_existing_vc_file("${dir}" ${name}_vc)
+  endif()
 
   # Create custom target to generate the VC revision include.
   set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/${name}Revision.inc")