Makefile: Always create clean target command

Borland Make complains if the phony clean target has no rule to build it.
If there are no files to clean, generate and run an empty clean script.
cmListFileLexer: Allow command names with one letter (#14181)

Teach the lexer to treat a single letter as an identifier instead of an
unquoted argument.  Outside of a command invocation, the parser treats
an identifier as a command name and an unquoted argument as an error.
Inside of a command invocation, the parser treats an identifier as an
unquoted argument.  Therefore this change to the lexer will make what
was previously an error case work with no other behavioral change.
cmListFileLexer: Allow a leading UTF-8 Byte-Order-Mark (#11137)

Teach the lexer to read a UTF-8, UTF-16 BE/LE, or UTF-32 BE/LE
Byte-Order-Mark from the start of a file if any is present.  Report an
error on files using UTF-16 or UTF-32 and accept a UTF-8 or missing BOM.
cmListFileLexer: Convert CRLF -> LF newlines explicitly

Read input files in binary mode instead of text mode and convert CRLF
newlines to LF newlines explicitly in our own buffer.  This is necessary
to read CMake source files with CRLF newlines on platforms whose C
runtime libraries do not transform newlines in text mode.  For example,
a Cygwin or Linux binary may not transform CRLF -> LF in files read from
a Windows filesystem.  Perform the conversion ourselves to ensure that
multi-line string literals in CMake source files have LF newlines
everywhere.
Add Lua-style long brackets and long comments to CMake language

Teach the CMake language parser to recognize Lua-style "long bracket"
arguments.  These start with two '[' separated by zero or more '='
characters e.g. "[[" or "[=[" or "[==[".  They end with two ']'
separated by the same number of '=' as the opening bracket.  There is no
nesting of brackets of the same level (number of '=').  No escapes,
variable expansion, or other processing is performed on the content
between such brackets so they always represent exactly one argument.

Also teach CMake to parse and ignore "long comment" syntax.  A long
comment starts with "#" immediately followed by an opening long bracket.
It ends at the matching close long bracket.

Teach the RunCMake.Syntax test to cover long bracket and long comment
cases.
MSVC: Add /FS flag for cl >= 18 to allow parallel compilation (#14492)

In generators such as Ninja that can run multiple "cl" processes that
refer to the same compiler .pdb file (/Fd) at the same time, MSVC from
Visual Studio 2013 complains:

 fatal error C1041: cannot open program database '.../vc120.pdb';
 if multiple CL.EXE write to the same .PDB file, please use /FS

According to "cl /?":

 /FS force to use MSPDBSRV.EXE

Add the flag to compilation lines for this compiler version just after the
/Fd option.
include(): Use lower case and () to refer to the include() command.
cmPolicies: Fix typo

Use American spelling: behaviour -> behavior.
target_link_libraries(): Fix code snippet in documentation.

The porting script seems to have made a mistake here.
cmTarget: Remove unused variable.
Help: Add introduction section to cmake-policies manual

Explain the concept behind policies.
cmExtraEclipseCDT4Generator: Replace RunCommand with RunSingleCommand

Avoid use of the old cmSystemTools::RunCommand by using RunSingleCommand
instead.
cmake: Drop "cmake -E comspec" Win9x helper

Since we dropped Win9x support there is no need for this helper.
exec_program: Re-implement using KWSys Process

Drop use of cmSystemTools::RunCommand.  It used popen on UNIX
(equivalent to /bin/sh -c "$command") and direct CreateProcess calls on
Windows.  Implement equivalent behavior using the KWSys Process library.
Copy windows shortpath conversion logic from cmSystemTools::RunCommand.
cmSystemTools: Drop old RunCommand method

All calls to this method have been replaced by newer infrastructure.
Remove it and the supporting cmWin32ProcessExecution class.
cmCoreTryCompile: Fix typo in comments.

Add 'S' to match actual variable names.
CMake Nightly Date Stamp
Merge topic 'cmake-syntax-updates'

daa0f6f Add Lua-style long brackets and long comments to CMake language
a8c6523 cmListFileLexer: Convert CRLF -> LF newlines explicitly
dbd9333 cmListFileLexer: Allow a leading UTF-8 Byte-Order-Mark (#11137)
5645783 cmListFileLexer: Allow command names with one letter (#14181)
Merge topic 'vs12-parallel-cl-FS'

216afc8 MSVC: Add /FS flag for cl >= 18 to allow parallel compilation (#14492)
Merge topic 'policies-generic-docs'

6d50d01 Help: Add introduction section to cmake-policies manual