CMake Nightly Date Stamp
Tests: Fix name of cache variable.
Help: Fix order of help entries.
Project: Remove extern from static string in feature tests.
Project: Use nullary form of main for compile feature tests.
Project: Fix exit-on-error with compile feature tests.
Merge topic 'minor-cleanups' into next

c4f4dac2 Project: Fix exit-on-error with compile feature tests.
5bb7ce72 Project: Use nullary form of main for compile feature tests.
64254e7a Project: Remove extern from static string in feature tests.
0d9c99bf Help: Fix order of help entries.
dc7639bd Tests: Fix name of cache variable.
c010e7cc CMake Nightly Date Stamp
Features: Decay language flag if requested is not available.

Use the highest standard compile flags available if requested language
version is too new.

This supports use-cases like

 set(CMAKE_CXX_STANDARD 14)

 # Compiled with -std=c++11 with GNU 4.7, which has no -std=c++14
 # or equivalent flag
 add_executable(main main.cpp)

This can be used in combination with preprocessor defines which
communicate the availability of certain language features for
optional use.
cmTarget: Add CXX_STANDARD_REQUIRED to control decay.
Merge topic 'decay-language-version' into next

205215fb cmTarget: Add CXX_STANDARD_REQUIRED to control decay.
1df2116b Features: Decay language flag if requested is not available.
Features: Record for GNU 4.7.

Update the CompileFeatures test to verify that unsupported features
do not work.
Merge topic 'GNU-4.7-features' into next

d9b31156 Features: Record for GNU 4.7.