Use `export-subst` to get information about the Git commit used to
produce an archive.

Update `.clang-format` with configuration to make the 6.0 format as
close as possible to what 3.8 produced before.  Then revise the style:

* Indent preprocessor directives (a feature new since 3.8)
* Add a newline and indentation before inheritance `:` and `,`

Rename the Git attribute identifying the format to include the
clang-format version number: `format.clang-format-6.0`.  This will aid
external infrastructure in knowing what version of the tool to run.

Daniel Pfeifer authored 8 years ago and Brad King committed 8 years ago

Revise the manual procedure in the `*Lexer.in.l` files.  Some of our
post-processing steps are no longer necessary with the current set of
supported compilers.  Some steps changed with newer versions of flex.

Then regenerate all lexers with flex version 2.6.1.

All clients of `cm_sha2` have been ported to `cmCryptoHash`, which now
uses librhash internally.

Revise the documented modifications we need to make to the
flex-generated source file according to the needs of the new version.
Also drop the duplicate copyright notice block from the generated file.

Revise the documented modifications we need to make to the
flex-generated source file according to the needs of the new version.
Update our own implementation to avoid warnings with flex types.

Copy cm_sha2.[hc] from sha2.[hc] in "sha2-1.0.tar.gz" downloaded today
from

  http://www.aarongifford.com/computers/sha.html

with trivial whitespace cleanup.  Also fix #include to account for
rename.