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This fixes #1214

Fixes #1215

Introduce support for DJGPP cross compiler

DJGPP cross compiler is targeting DOS which does not support POSIX
signals. Probably for the same reason (targeting DOS) this compiler
does not support wide characters.

The support is turned on by default but the user might need to be able
to turn it off which is now possible by defining CATCH_CONFIG_NO_WCHAR.

The old version would lead to error when Catch was installed
as a subproject. The file would be written to the subproject's
build directory and then would not be installed properly.

* Examples are no longer built for all images
* Coverage is no longer collected from every build
* The number of configurations is reduced

* Examples are no longer built on all travis images
* Coverage is no longer collected from all travis images
* Valgrind is no longer used with all travis images

This should greatly reduce the amount of compiling, downloading
binaries and general work the common images do.

Needed to embed newer version of Clara

Closes #1178
Closes #1031

* Eliminated some copies
* Made makeTestCase fit into 4 arguments -- avoids spills on Win64
* Made string literals into StringRef literals

The warning occurred when !CATCH_CONFIG_WINDOWS_SEH
&& !CATCH_CONFIG_POSIX_SIGNALS.

This allows reducing the amount of friends needed for its interface
and some extra tricks later.

The bad part is that the pointer can become invalidated via
calls to other StringRef's public methods, but c'est la vie.

Also fix how it can be disabled.

It was a bit of a mess previously

To prevent bugs with stitching system headers inside Catch,
the proxy header is responsible for guarding against inclusion
on Linux, rather than the includers.

Might be related to #1197

This also fixes some tests that were previously failing unnoticed - WTF?

This allows reuse of projects existing stringification machinery

Closes #1024

Related to #1114

Related to #1183

Closes #1158