ExternalData: Allow local stores without any URL templates

Allow ExternalData_URL_TEMPLATES to be empty if a value for
ExternalData_OBJECT_STORES is provided.  Assume in this use case that
the object stores will already contain all needed objects.  Extend the
Module.ExternalData test to cover this case (all objects in stores).
Extend the RunCMake.ExternalData test to cover the non-failure message
case when stores are provided without URL templates.
Drop use of configure_file IMMEDIATE option

Since commit 7d47c693 (Drop compatibility with CMake < 2.4, 2013-10-08)
we no longer need to use the configure_file IMMEDIATE option to support
compatibility modes less than 2.0.
set: Handle value-less PARENT_SCOPE explicitly

The code "set(VAR PARENT_SCOPE)" unsets VAR in the parent scope.
Implement this case explicitly next to where "set(VAR)" is explicitly
handled to unset VAR in the current scope.
set: Fix handling of empty value with PARENT_SCOPE

Just as

  set(VAR "")

sets VAR to an empty string in the current scope, the code

  set(VAR "" PARENT_SCOPE)

is documented to set the variable to an empty string in the parent
scope.  Fix the implementation to make it so.
set: Add unit tests for set/unset PARENT_SCOPE

Create a RunCMake.set test to cover set() command cases, starting with
PARENT_SCOPE.
FindOpenMP: fix detecting compilers that do not need any special flag (#14567)
Help: Format manual/*.1.rst options as definition lists

Convert the command-line option documentation formatting from bullet
lists to definition lists.
MSVC: Fix encoding of Visual Studio 10+ project files.

Use Windows-1252, which is the same encoding as older
Visual Studio project files.
GetPrerequisites:  Preserve search path order from caller.
Merge topic 'ExternalData-allow-no-urls'

8eb20ee ExternalData: Allow local stores without any URL templates
Merge topic 'configure_file-drop-IMMEDIATE'

15610bb Drop use of configure_file IMMEDIATE option
Merge topic 'set_emptyvar_PARENT_SCOPE'

bf755c7 set: Add unit tests for set/unset PARENT_SCOPE
bc280f1 set: Fix handling of empty value with PARENT_SCOPE
20afbd5 set: Handle value-less PARENT_SCOPE explicitly
Merge topic 'msvc-encoding'

58e3d49 MSVC: Fix encoding of Visual Studio 10+ project files.
Merge topic 'openmp-cray'

ebf70b0 FindOpenMP: fix detecting compilers that do not need any special flag (#14567)
Merge topic 'gp-ld_library_path'

e130b9e GetPrerequisites:  Preserve search path order from caller.
Merge topic 'doc-tool-options-as-definition-lists'

3442f2b Help: Format manual/*.1.rst options as definition lists