tests: Add variable expansion tests

There are some corner cases in variable expansion which would be nice to
capture before going and rewriting the variable expansion code.
CMake Nightly Date Stamp
Ninja: Cache target-level flags

Instead of figuring out target flags per-source file, cache the flags
that are being used. This results in a *much* faster generate time for
Ninja.
Windows: Make file delete/rename retry configurable

Several CMake operations need to replace files in rapid succession.
This commonly fails on Windows due to filesystem lock behavior so
we have retry loops.  No matter how many times we retry or how long
we delay there will inevitably be someone with an environment that
needs more.  Make the retry count and delay configurable in the
Windows Registry keys:

 {HKCU,HKLM}/Software/Kitware/CMake/Config

in DWORD values

 FilesystemRetryCount = Number of tries
 FilesystemRetryDelay = Delay in milliseconds between tries

Leave the feature undocumented for now to see how it goes.
Merge topic 'windows-filesystem-retry-config' into next

7b1f966a Windows: Make file delete/rename retry configurable
5104f55d CMake Nightly Date Stamp
OS X: Escape path given to -isysroot flag

Ensure that paths containing spaces or other special characters are
escaped correctly on the command line.
Add policy CMP0049 to avoid variable expansion in source lists
add_custom_command: Disallow use of SOURCE signatures.

Add CMP0050 to control this behavior.
Merge topic 'osx-sysroot-escape' into next

77b2e6f1 OS X: Escape path given to -isysroot flag
Help: Fix wording of CMP0049 and CMP0050 release notes

Also tweak description of policy CMP0049 to clarify that source
lists were not expanded, just variables in individual source names.
Merge topic 'backward-compatibility' into next

ddd0db99 Help: Fix wording of CMP0049 and CMP0050 release notes
Merge topic 'backward-compatibility' into next

f2eee72f add_custom_command: Disallow use of SOURCE signatures.
c248a437 Add policy CMP0049 to avoid variable expansion in source lists
Merge topic 'FindBoost-MPI-hints'

a22eeca3 FindBoost: Search next to MPI libraries for boost_mpi (#14739)
Merge topic 'osx-intel-info-split'

3693c63f OS X: Split Intel compiler information files
Merge topic 'cmake-devel-version-macro'

6ca6613a Export: Use the CMAKE_DEVEL_VERSION macro for build-export files.
Merge topic 'dev/fix-sublime-compile-flags'

2d9f93ab Sublime: Pass language and config properly
Merge topic 'backward-compatibility'

f2eee72f add_custom_command: Disallow use of SOURCE signatures.
c248a437 Add policy CMP0049 to avoid variable expansion in source lists
Merge branch 'master' into next
Revert custom-parsers

Too many corner cases :( .
Merge topic 'revert/custom-parsers' into next

fead0cff Revert custom-parsers