ExternalProject: Always add a command to a step

Some generators (i.e. Xcode) will not generate a file level target if
no command is set, and therefore the dependencies on this target will
be broken.
This patch sets an empty echo command that does nothing to avoid this
issue.
ExternalProject: Add UPDATE_DISCONNECTED option

If UPDATE_DISCONNECTED is set, the update step is not executed
automatically when building the main target. The update step can still
be added as a step target and called manually. This is useful if you
want to allow to build the project when you are disconnected from the
network (you might still need the network for the download step).
This is disabled by default.

The directory property EP_UPDATE_DISCONNECTED can be used to change
the default value for all the external projects in the current
directory and its subdirectories.
ExternalProject: Add unit tests for UPDATE_DISCONNECTED
FindIntl: New module to find Gettext libintl

Add support for Sun/Uniforum/GNU gettext libintl.  This belongs in CMake
rather than upstream because:

* There are multiple upstreams (Sun, GNU).

* It may or may not be in the glibc C library depending
  upon the platform and build options used.

Although we already have a FindGettext module, that is for the tools.
This module is for the library, and is independent because it's
perfectly OK to use libintl without the gettext tools (and vice versa),
and they might not all be found.  Add cross references between the two
modules in notes to make the relationship clearer.
Refactor and seperate search path construction for find commands

Prior to this commit, the set of search paths to traverse for find commands
was incrementally constructed.  This change allows each group of paths, i.e.
CMakeVariablePaths, UserHintsPaths, SystemEnvironmentPaths, etc. to be
constructed and manipulated independently, and then all combined togethor.
Encapsulate search path manipulation functions into a seperate class.

The functions for adding the various different types of paths have been
factored out into a new class, cmSearchPath.  It is to be used as a helper
container class for the various find_* commands.
Genex: Fix whitespace issue.
Features: Split the Clang testable features to a separate file.

These tests can be shared with the AppleClang compile feature tests.
Use containers of labeled search paths instead of individual members

Manage classes of search paths in labeled containers.  This removes the
need to have a seperate member variable for each type of search path, but
also allows path types to be grouped togethor in various different ways
and manipulated as subsets of the full set of search paths.
CMake 3.1.0-rc2
file(GENERATE): Evaluate early to allow generating source files

The evaluation files must be known before cmTargetTraceDependencies
attempts to find them, but we must actually generate the files after
cmTargetTraceDependencies, as that can add to target SOURCES.  The
limitation is that the generated output name must not depend on the
SOURCES of a target if the generated file is used by that target.

Mark the output files as GENERATED so that trace dependencies does
not expect them to already exist in the filesystem.

Move the invokation of ForceLinkerLanguage in the Generate logic
to after the generated file names are known.  ForceLinkerLanguage
tries to determine the sources of a target (in order to determine
an already-known language) and otherwise fails to get information
about the generated file.

Test that the output of file(GENERATE) can be used as a target source
file and that accessing the target SOURCES in the name of the output
file is an error.  Accessing the TARGET_OBJECTS would be a similar
error if it was legal to use that generator expression in this
context.  That is not currently possible and is a different error
condition, so test the current error output as a reminder to change
the expected output if that becomes possible in the future.  Test
that generated rule files resulting from cmTargetTraceDependencies
appear in the SOURCES generated in the output file.
Features: Reorder the GNU C dialect feature tests

This doesn't make a difference, but the consistency with other files
is easier to reason about.
Features: Use the correct dialect flag when recording features.

Avoid using -std=c++1y for compilers which support -std=c++14, for
example.
Features: Run GNU feature tests with std=c++14 when available.
Features: Don't record for AppleClang

Features are currently recorded accidentally for all versions of
AppleClang > 3.4 (I have no idea how that relates to upstream
Clang). Presumably that version has the features which are
accidentally recorded, but in the future features will be
recorded initially for only AppleClang >= 5.1, which would
appear as a feature regression.

Commit v3.1.0-rc1~635^2~11 (Don't load Clang-CXX from
AppleClang-CXX., 2013-11-11) ajusted the logic for the CXX
language.  Make a similar change for the C language.
Help: Document COMPONENT option of install(SCRIPT/CODE) (#14956)
Help: Add notes for topic 'ExternalProject_UPDATE_DISCONNECTED'
Merge topic 'ExternalProject_UPDATE_DISCONNECTED'

3c497f11 Help: Add notes for topic 'ExternalProject_UPDATE_DISCONNECTED'
aba5cec6 ExternalProject: Add unit tests for UPDATE_DISCONNECTED
3f606fa7 ExternalProject: Add UPDATE_DISCONNECTED option
c0b749cf ExternalProject: Always add a command to a step
Merge topic 'doc-install-SCRIPT-CODE-COMPONENT'

7361e810 Help: Document COMPONENT option of install(SCRIPT/CODE) (#14956)
Merge topic 'refactor-search-path-construction'

1abd7cd9 Use containers of labeled search paths instead of individual members
2a9ac4bd Encapsulate search path manipulation functions into a seperate class.
32922840 Refactor and seperate search path construction for find commands