Add FindBacktrace.cmake module.

It designed to search for implementation of backtrace(3) routine.
Currently it is used in OpenBSD Ports for building Clementine
music player.

A lot of input from brad.king@ and neundorf@.
Credit Kitware and csimsoft for their maintenance roles

Add maintenance credit to command-line "--version" output and the
cmake-gui "About" dialog.
FindwxWidgets: Add support for wxWidgets 3.0.0 (#14587)

Add the "wxWidgets-3.0.0" path suffix and the "30" library names.
While at it, add the "wxWidgets-2.9.5" suffix too.
Add policy CMP0040 to disallow custom commands on missing targets
Merge topic 'find_backtrace'

c6f339c Add FindBacktrace.cmake module.
Merge topic 'FindwxWidgets-3.0.0'

2cd5597 FindwxWidgets: Add support for wxWidgets 3.0.0 (#14587)
Merge topic 'cmake-credits'

6fc4ab9 Credit Kitware and csimsoft for their maintenance roles
Merge topic 'missing-target-error'

a02f3d2 Add policy CMP0040 to disallow custom commands on missing targets
Help: Reformat cmake-generator-expressions.7 manual

Use definition lists instead of a preformatted block to enumerate
the genex documentation.

Consistently capitalize the description.

Use ``...`` to format genex parameters in the description.

Turn references to commands into links.

Add high level documentation about the motivation for generator
expressions.

Regroup expressions into subsections for different genex types. Add
a high-level description and example of the expressions in each
section.

Explode the documentation for filesystem artifacts, instead of only
referring to the variations.
Merge topic 'genex-docs'

8e729df Help: Reformat cmake-generator-expressions.7 manual