Sane (un)install for empty directory variables.

Sane (un)install for empty directory variables.

This patch fixes all install and uninstall rules to behave
sanely when $(wheredir) is the empty string, but $(where_HOW) is
nonempty, for all kinds of values of `where' and `HOW'.

* lib/am/data.am (install-%DIR%%PRIMARY%, uninstall-%DIR%%PRIMARY%):
Do not touch the install tree if the corresponding directory variable
$(%NDIR%dir) is empty.
* lib/am/java.am (install-%DIR%JAVA, uninstall-%DIR%JAVA): Likewise.
* lib/am/libs.am (install-%DIR%LIBRARIES, uninstall-%DIR%LIBRARIES):
Likewise.
* lib/am/lisp.am (install-%DIR%LISP, uninstall-%DIR%LISP): Likewise.
* lib/am/ltlib.am (install-%DIR%LTLIBRARIES, uninstall-%DIR%LTLIBRARIES):
Likewise.
* lib/am/mans.am (install-man%SECTION%, uninstall-man%SECTION%):
Likewise.
* lib/am/progs.am (install-%DIR%PROGRAMS, uninstall-%DIR%PROGRAMS):
Likewise.
* lib/am/python.am (install-%DIR%PYTHON, uninstall-%DIR%PYTHON):
Likewise.
* lib/am/scripts.am (install-%DIR%SCRIPTS, uninstall-%DIR%SCRIPTS):
Likewise.
* lib/am/texinfos.am (install-dvi-am, install-html-am, install-pdf-am)
(install-ps-am, uninstall-dvi-am, uninstall-html-am, uninstall-pdf-am)
(uninstall-ps-am): Likewise.
* tests/instdir2.test, tests/instdir-java.test,
tests/instdir-lisp.test, tests/instdir-ltlib.test,
tests/instdir-prog.test, tests/instdir-python.test,
tests/instdir-texi.test: New tests.
* tests/Makefile.am: Update.
* NEWS: Update.
Suggestion by Akim Demaille.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>