Build CMake with most-recent available language dialect.

When CMake is built with CMake 3.1 or later, appropriate -std=
options will be added for GNU and Clang compilers while building
C and CXX code.

This allows taking advantage of 'hidden' language features such
as move-constructors, and allows the standard library to enable
the use of more-advanced features too, where available.

This does not change CMake host compiler requirements.
FindwxWidgets: Add versions 3.0.1 and 3.0.2
Merge branch 'FindwxWidgets-new-versions' into release
Revert "ClearMatches: Only clear matches which were actually set" (#15261)

This reverts commit v3.1.0-rc1~557^2~2 (ClearMatches: Only clear matches
which were actually set, 2014-03-12).  The optimization did not track
the match count in the same scope as the variables, allowing possible
inconsistency.

Resolve conflicts in Source/cmIfCommand.cxx, Source/cmMakefile.cxx,
and Source/cmMakefile.h by moving the changes to the new location
of the code involved.
Merge branch 'revert-cached-regex-clear' into revert-cached-regex-clear-for-master

Resolve conflict in Source/cmMakefile.h by integrating both changes.
Merge branch 'revert-cached-regex-clear' into release
Genex: Fix evaluation context propagation for TARGET_OBJECTS.

Extract a new method to encapsulate the requirements of evaluating
dependent-expressions, namely, propagation of the
EvaluateForBuildsystem setting, which is missing from the
getLinkedTargetsContent implementation.

Commit v3.1.0-rc1~688^2 (Genex: Only evaluate TARGET_OBJECTS to determine
target sources., 2014-03-20) introduced an error case for use of
TARGET_OBJECTS outside of the context of generating the buildsystem,
as the path to object files may be dependent on buildsystem
variables (See bug #15226).

Commit v3.1.0-rc1~314^2 (Allow INTERFACE_SOURCES to specify
$<TARGET_OBJECTS> (#14970), 2014-07-09) made it possible to
propagate such content to dependent targets.

While that commit propagated the EvaluateForBuildsystem setting
for the case of a TARGET_PROPERTY expression, as generated for
direct dependencies of a target in
cmTargetInternals::AddInterfaceEntries, it did not add propagation
for content from further transitive target dependencies, as determined
by getLinkedTargetsContent.
Fix lookup of source names after conversion to their actual case (#15259)

Since commit v3.1.0-rc1~688^2~15 (cmTarget: Add a method to obtain list of
filenames for sources, 2014-03-17) we have code paths that lookup sources by
strings containing their own full path after normalization to the actual case
on disk.  This fails in the case that a cmSourceFile has already been created
with a different case in the filename.  The comparison of the directory works
because it is always normalized.  Only the comparison of the file name fails.

Fix this by using a case-insensitive comparison of source file names on
platforms that do not have case-sensitive filesystems.
Merge branch 'fix-source-case-matching' into release
cmTarget: Don't allow relative paths in INTERFACE_SOURCES

Follow the pattern of checks that are made for INTERFACE_INCLUDE_DIRECTORIES.
Existence is already checked by cmSourceFile::GetFullPath.  Add a check
to disallow relative paths in source directories. Otherwise code such as

 target_sources(lib1 INTERFACE foo.cpp)

would fail if consumed by a target in a different directory.

Unlike the INTERFACE_INCLUDE_DIRECTORIES behavior, we don't care whether
the entry comes from an IMPORTED target or not.  In the include directories
case, the directory for a non-imported target might not exist yet but
might be created.  In the sources case, a file which does not yet
exist in the filesystem must be explicitly marked with the GENERATED
property.

Adjust existing tests and add a new test for the error.
Export: Disallow export of targets with INTERFACE_SOURCES

This can be allowed in the next release, but it needs to have some
features present and tested such as

* Ensuring that relative paths do not appear in the generated property.
* Ensuring that paths to the source or build directories do not appear.
* Generating a check in the file for CMake 3.1 or later so that the
    resulting property will be consumed.
* Ensuring that any referenced targets are part of an export set and
    generating a check for them.
* INSTALL_INTERFACE and BUILD_INTERFACE content.

All of these checks are already done for INTERFACE_INCLUDE_DIRECTORIES,
but it is too late to add them for INTERFACE_SOURCES for CMake 3.1.
As the checks introduce some new error conditions, it is better to
disallow exporting fully for this case and introduce proper error
conditions later instead of policies.
Help: Document the export limitation of INTERFACE_SOURCES.
CMake Nightly Date Stamp
Help: Remove duplicate '#' in CMP0053 valid character list
Help: Format set_property and get_property command docs
Help: Document installed file property API

Since commit v3.1.0-rc1~479^2~1 (Add an "installed file" property
scope, 2014-05-15) the get_property and set_property commands
support an 'INSTALL' scope.  Add documentation for this scope.
Help: Add 3.1 release notes for INSTALL property scope
Merge branch 'doc-CMP0053-dedup-char' into release
Merge branch 'doc-installed-file-property' into release
Merge branch 'fix-transitive-OBJECT_SOURCES-context' into release