ExternalProject: Find VCS packages quietly

This keeps the VCS packages out from FeatureSummary output, unless
the INCLUDE_QUIET_PACKAGES is used.
CMake Nightly Date Stamp
CMake Nightly Date Stamp
Simplify.
Merge topic 'optimize-source-file-processing' into next

1f6edef6 Simplify.
cmTarget: Extract a ProcessSourceItemCMP0049 method.

Avoid calling AddSource for each src filename.  That involves
checking each entry for uniqueness and creating a separate
generator expression for each one.

Instead, add a single entry for the list of sources.  The source
files are passed through a uniqueness filter at generate-time, so
duplicates don't matter so much.
cmTarget: Cache the cmSourceFiles in GetSourceFiles.

Avoid calling GetSourceFiles with the same result container multiple
times when tracing target dependencies.  The result from the previous
configuration is cached and used later otherwise.
cmGeneratorTarget: Trace cmSourceFile objects instead of strings.

This reverses the decision in commit d38423ec (cmTarget: Add a
method to obtain list of filenames for sources., 2014-03-17).  The
cmSourceFile based API is preferred because that avoids creation of
many cmSourceFileLocation objects for matching strings, and the
result is cached by cmTarget.
cmTarget: Make GetSourceFiles string overload private.

Consumers should use the cmSourceFile overload, which is now
always the case.
Merge topic 'optimize-source-file-processing' into next

a4e6bf8e cmTarget: Make GetSourceFiles string overload private.
92e2fbe1 cmGeneratorTarget: Trace cmSourceFile objects instead of strings.
c5b26f3b cmTarget: Cache the cmSourceFiles in GetSourceFiles.
eb163f37 cmTarget: Extract a ProcessSourceItemCMP0049 method.
Features: Add cxx_inline_namespaces.
Features: Expect cxx_variadic_templates to implement N2555.

N2555 is essentially a bugfix and predates most compiler releases which
aimed to experimentally support variadic templates.
Features: Add cxx_thread_local.
Features: Add cxx_sizeof_member.
Features: Add cxx_decltype_incomplete_return_types.
Features: Add cxx_right_angle_brackets.
Use a cxx_attributes test that does not produce a warning.
Fix extern templates version.
Allow variadic macros only in C++11 mode for now.
Features: Add cxx_default_function_template_args.