Modify --with-pic to support per-package configurations.

Modify --with-pic to support per-package configurations.

* libltdl/m4/libtool.m4:  Modify --with-pic to accept a list of
package names.  Modelled off --enable-shared.
* tests/with-pic.at: New test.
* Makefile.am (TESTSUITE_AT): Add tests/with-pic.at.
* doc/libtool.texi (LT_INIT): Enhance documentation of
--with-pic configure flag.
* NEWS (New features): Mention that --with-pic now accepts a
comma-separated list of package names.

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