Issue 5739: Add makefile targets for formatting all C++ code

https://sourceforge.net/p/testlilyissues/issues/5739/

These are not yet intended for routine use by contributors.  They are
meant to help explore the differences between astyle and clang-format.

[dak, 2020-02-05 17:52:20]

https://codereview.appspot.com/565620043/diff/549530043/GNUmakefile.in
File GNUmakefile.in (right):

https://codereview.appspot.com/565620043/diff/549530043/GNUmakefile.in#newcod...
GNUmakefile.in:319: ## TODO: This condition speeds up make when formatting
targets are not
I would not really work with a timestamp file but rather use git status to
detect modified files, so that only those would be affected.  That, however, is
more a useful approach for a script rather than a Makefile.

https://codereview.appspot.com/565620043/diff/549530043/configure.ac
File configure.ac (right):

https://codereview.appspot.com/565620043/diff/549530043/configure.ac#newcode367
configure.ac:367: STEPMAKE_PROGS(CLANG_FORMAT, clang-format-9 clang-format,
OPTIONAL, 9, 9)
Interesting.  This gives a warning when it isn't installed?  I think that we
don't usually flag dependencies of components not involved in either building or
running LilyPond.  For example, we provide Emacs and vi style files without
checking for availability of either editor.

[Dan Eble, 2020-02-05 19:23:04]

On 2020/02/05 17:52:20, dak wrote:
> configure.ac:367: STEPMAKE_PROGS(CLANG_FORMAT, clang-format-9 clang-format,
> OPTIONAL, 9, 9)
> Interesting.  This gives a warning when it isn't installed?  I think that we
> don't usually flag dependencies of components not involved in either building
or
> running LilyPond.  For example, we provide Emacs and vi style files without
> checking for availability of either editor.

Yes, it gives a warning.  If this is a problem, IMO its is a shortcoming of the
configuration system: failing to distinguish between simply optional programs
and strongly recommended programs.  There's a similar issue with tidy.  If it's
there, we want to take advantage of it, but warning the dev that it is missing
is a nuisance.