Merge branch 'maint'

Merge branch 'maint'

* maint:
  docs: document planned precedence changes in aclocal search path
  aclocal: handle ACLOCAL_PATH environment variable
  tests: fix spurious failure in 'primary-prefix-valid-couples.test'
  docs: deprecate JAVA primary
  docs: clearer distinction between `.java' with javac and with gcj
  java: complain if java_JAVA is used but $(javadir) is undefined