FindThreads: move checking of the -pthread compiler flag into a macro

This allows a following commit to introduce a switch to prefer that check over
searching for the explicit library names without breaking backward
compatibility.
Ninja: Limit custom command side-effects to build folder

Revert commit 7243c951 (Ninja: Don't limit custom cmd side-effects to
build folder, 2014-06-27) because it causes every custom command
dependency in the source tree to get a phony rule.  For large projects
these rules get too big for Ninja to handle efficiently.  While the
original change addressed a valid concern, it did not seem to occur
regularly in practice because well-behaved projects generate their
side-effects only in the build tree.  Until we support explicit
specification of side-effects (CMake issue #14963), we will have to use
this as a middle-ground.
Help: Replace 'Dart' with 'CDash' in ctest.1 manual

The modern dashboard server is CDash, not Dart, so update the manual
to talk about CTest as a CDash client.  Also drop documentation for
a Dart2-specific option.
FindThreads: introduce an imported target to link to

This not only holds the library, but can also hold compiler flags needed, e.g.
the -pthread flag preferred by gcc on some platforms. There was no clean way
to get that compiler flag from the module until now.
FindThreads: introduce THREADS_PREFER_PTHREAD_FLAG (#14767)
Merge topic 'ninja-phony-rules-only-in-build-tree'

de8e534b Ninja: Limit custom command side-effects to build folder
Merge topic 'doc-drop-dart'

b8aa0cdf Help: Replace 'Dart' with 'CDash' in ctest.1 manual
Merge topic 'FindThreads_overhaul'

b7e5c5a2 FindThreads: introduce THREADS_PREFER_PTHREAD_FLAG (#14767)
bcb0e387 FindThreads: introduce an imported target to link to
46368edd FindThreads: move checking of the -pthread compiler flag into a macro