Issue 5572: Compile modified C++ files automatically before testing (issue 553080043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
https://sourceforge.net/p/testlilyissues/issues/5572/

Issue 5572/1: Compile modified C++ files automatically before testing

The main change is to make sure lilypond and scripts are up to date
before running tests.  Build fonts too, though this is only sufficient
to handle builds following a `make clean`.

Other changes:

* Make a single baseline copy of fonts instead of replicating them in
  every subdirectory with tests.

* If the developer has forgotten to create a baseline, try to catch it
  quickly.

* Fix warnings about multiple sets rules for for local-test by
  renaming one of the targets lysdoc-test.

Issue 5572/2: Contributor's Guide updates

Please review this at https://codereview.appspot.com/553080043/

Affected files (+105, -67 lines):
  M Documentation/contributor/introduction.itexi
  M Documentation/contributor/programming-work.itexi
  M Documentation/contributor/regressions.itexi
  M GNUmakefile.in
  M make/lysdoc-targets.make
  M stepmake/stepmake/generic-vars.make
  M stepmake/stepmake/texinfo-rules.make