build: Fallback $docdir to $datarootdir/doc/gettext

Problem reported by Santiago Vila in
<http://lists.gnu.org/archive/html/bug-gettext/2014-05/msg00014.html>.
* gettext-runtime/configure.ac: Adjust $docdir assuming that
AC_PACKAGE_TARNAME is set.
* gettext-tools/configure.ac: Likewise.
* gettext-tools/examples/configure.ac: Likewise.
tests: Improve test coverage
filter-quote: Minor refactoring of string copy
msgfmt: Report error on accelerator mismatch

* msgl-check.c (check_pair): Increment error count on missing
accelerator character or too many accelerators.
Update .gitignore
tests: Fix errors when CFLAGS contains --coverage

* lang-c: Supply $CFLAGS to the libtool link command.
* lang-objc: Likewise.
Update Gnulib
Update after gnulib changed.
Remove non-existing regex.diff from distribution
Fix typo in commit 2a644d7a
tests: Don't assume ga_IE locale is present

* msginit-3: Fix charset value after msginit invocation.
msgexec: Implement plural support

* msgexec.c (process_string): Set or unset MSGEXEC_MSGID_PLURAL.
(process_message): Set or unset MSGEXEC_PLURAL_FORM.
msgfilter: Implement plural support

* msgfilter.c (process_message): Set or unset
MSGFILTER_MSGID_PLURAL and MSGFILTER_PLURAL_FORM.
msgexec, msgfilter: Fix typo in comments

Refere to the correct environment variables MSG{EXEC,FILTER}_MSGCTXT
and MSG{EXEC,FILTER}_MSGCTXT in comments.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>
tests: Add test for msgexec plural handling
tests: Add test for msgfilter plural handling
NEWS: Mention msgexec/msgfilter plural handling
build: Simplify man and HTML documentation generation

Problem reported by Santiago Vila in:
<https://lists.gnu.org/archive/html/bug-gettext/2014-05/msg00024.html>.
build: Fix VPATH build for po-gram-gen2.h
msgfilter: Pass previous msgid to the child process

Suggested by Pavel Kharitonov in:
<http://lists.gnu.org/archive/html/bug-gettext/2014-05/msg00022.html>.
* msgfilter.c (process_message): Set MSGFILTER_PREV_* envvar.