Fix parallel make race when generating unistring/cdefs.h.

Trivial fix by Richard Purdie in:
http://lists.openembedded.org/pipermail/openembedded-core/2012-April/059850.html
* lib/Makefile.am (unistring/cdefs.h): Add $(UNUSED_PARAMETER_H)
to prerequisites.
Reported by Ludovic Courtès.
Replace deprecated $(mkdir_p) usage.

Problem report and trivial fix by Nitin A Kamble in:
https://lists.gnu.org/archive/html/bug-libunistring/2012-07/msg00000.html
* doc/Makefile.am (installdirs-dvi, installdirs-ps)
(installdirs-pdf, install-html-monolithic, install-html-split)
(installdirs-html, dist-html-monolithic, dist-html-split): Use
$(MKDIR_P) instead of deprecated $(mkdir_p).