Help: Add install() command document section headers

Use section headers instead of horizontal dividers so that one may link
to the sections.
cmInstallTargetGenerator: Drop default constructor arguments

They are used only in cmLocalGenerator::GenerateTargetInstallRules.
Move the defaults to a local helper where the context justifies their
values.
file(INSTALL): Do not pre-create DESTINATION for DIRECTORY

When installing a DIRECTORY, do not pre-create the DESTINATION.  The
cmFileCopier::InstallDirectory method will create the directory anyway.
Give it a chance to detect whether the directory already exists or not.
file(INSTALL): Report existing DIRECTORY as Up-to-date

Teach cmFileCopier::InstallDirectory to detect whether the destination
directory exists.  If so, report it as "Up-to-date" instead of
"Installing".  This resolves message asymmetry with file installations.

Extend the RunCMake.file and RunCMake.install tests to check the
installation output on both the first and second run.

Suggested-by: J Decker <d3ck0r@gmail.com>
file(INSTALL): Add undocumented options to control output verbosity

Create options "MESSAGE_ALWAYS", "MESSAGE_LAZY", and "MESSAGE_NEVER" to
specify whether to print the "Installing" and "Up-to-date" messages.
Extend the RunCMake.file test with cases covering these options.
install: Thread message level setting through internal API

Create a cmInstallGenerator::MessageLevel enumeration for future use in
specifying install message verbosity.  Thread values of the type through
constructors and save the value as a member of cmInstallGenerator.
Use only a "MessageDefault" value for now.
install: Add CMAKE_INSTALL_MESSAGE variable (#13761)

Create a variable to allow users to control which installation
messages are printed.  In particular, provide a "LAZY" setting
that prints "Installing" messages but not "Up-to-date" messages.
This is desirable for incremental re-installations.

Suggested-by: J Decker <d3ck0r@gmail.com>
install(DIRECTORY): Add MESSAGE_NEVER option to avoid output (#13761)

Installing large directories, e.g., the output of a doxygen run, prints
one line per file resulting in too much noise in the build output.  Add
an option to the install(DIRECTORY) command to not print anything upon
make install.

Extend the RunCMake.install test with cases covering MESSAGE_NEVER
behavior of the install(DIRECTORY) command.

Suggested-by: Stefan Eilemann <Stefan.Eilemann@epfl.ch>
cmCPackGenerator: Avoid crash on undefined CMAKE_MAKE_PROGRAM

Since commit 6557382d (stringapi: Use strings for program
paths, 2014-02-24), cmGlobalGenerator::GenerateBuildCommand
takes the make program path as a std::string.  Fix the lookup
in cmCPackGenerator::InstallProjectViaInstallCMakeProjects to
use GetSafeDefinition for CMAKE_MAKE_PROGRAM to avoid possible
construction of std::string from a NULL.

Reported-by: Richard Wirth <richard@califax.de>
cmTarget: Teach ExpandLinkItems how to support $<LINK_ONLY>

Add a 'usage_requirements_only' parameter to ExpandLinkItems so that it
knows whether to use SetTransitivePropertiesOnly while evaluating
generator expressions.  Update existing call sites to pass 'false' since
they are for linking and not usage requirements.
cmTarget: De-duplicate link interface genex code for $<LINK_ONLY>

Simplify the implementation of GetTransitivePropertyTargets by using
ExpandLinkItems with usage_requirements_only==true to evaluate the
generator expressions in the link interface for us.
cmTarget: Teach GetLinkInterfaceLibraries to support $<LINK_ONLY>

Add a 'usage_requirements_only' parameter to GetLinkInterfaceLibraries
and supporting internal APIs to pass through to ExpandLinkItems so it
knows whether to use SetTransitivePropertiesOnly while evaluating
generator expressions.
cmTarget: De-duplicate link interface evaluation for $<LINK_ONLY>

Teach GetTransitivePropertyTargets to use the GetLinkInterfaceLibraries
method with usage_requirements_only==true instead of evaluating the
INTERFACE_LINK_LIBRARIES property directly.  This avoids duplicate
evaluations and makes use of the caching done by
GetLinkInterfaceLibraries.
cmTarget: Remove unnecessary 'mutable' markup

Members of the cmTargetInternals structure do not need to be made
'mutable' even to cache data because there is no reason for the
internal methods to be 'const'.
cmTarget: Refactor internal LinkImplementation map

If ComputeLinkImplementationLanguages were ever to cause
GetLinkImplementationLibraries to be invoked then a LinkImplMap entry
may appear in the middle of computing it in GetLinkInformation.  Instead
create the map entry up front and store in it boolean values indicating
which pieces of the LinkImplementation structure have been populated.
This approach leads to shorter code that is easier to follow too.
cmTarget: Shorten a long line in ComputeLinkImplementation

Prepare to change its indentation without exceeding line length limit.
cmTarget: Pre-indent a block in ComputeLinkImplementation

Prepare to move it into another block without extra whitespace changes.
cmTarget: Refactor ComputeLinkImplementation

Use LinkImplementationPropertyEntries directly instead of asking
GetProperty to construct a string for LINK_LIBRARIES.  This gives us
access to the entry backtraces.
CTestTestTimeout: increase the timeout for all platforms

Slow or busy machines not running Cygwin may also fail the 1 second timeout:

CMake Error at CTestTestTimeout/test.cmake:38 (message):
  Log does not exist:

    .../Tests/CTestTestTimeout/timeout.log

http://open.cdash.org/testDetails.php?test=264045338&build=3382733
http://open.cdash.org/testDetails.php?test=263904235&build=3382056
bootstrap: Clarify name of configured source directory

Rename CMAKE_ROOT_DIR to CMAKE_BOOTSTRAP_SOURCE_DIR to clarify both that
it is only for bootstrap and that it refers to the source directory.