maint: port to platforms lacking SIGPIPE

* tailor.h (SIGPIPE): Define to 0 if not defined.  This fixes a
porting bug introduced as part of 2012-11-16 syntax-check cleanup.
Problem reported by Bdale Garbee in
<http://lists.gnu.org/archive/html/bug-gzip/2013-06/msg00006.html>.
doc: zgrep exit status, unsupported options

* zgrep.1 (EXIT STATUS, BUGS): New sections.
Problem reported by Bdale Garbee in
<http://lists.gnu.org/archive/html/bug-gzip/2013-06/msg00007.html>.
tests: zgrep-context assumes grep knows context

* tests/Makefile.am (TESTS_ENVIRONMENT): Pass GREP too.
* tests/zgrep-context: Check that the underlying grep supports
context options.  Problem reported by Steven M. Schweda in
<http://lists.gnu.org/archive/html/bug-gzip/2013-06/msg00010.html>.
gzip: port util.c to Compaq C V6.5-303

* util.c (crc_32_tab): Move definition to front, since this
compiler doesn't allow declarations of static arrays with
incomplete types.  Problem reported by Steven M. Schweda in
<http://lists.gnu.org/archive/html/bug-gzip/2013-06/msg00010.html>.
zgrep: usage should say which grep options are not supported

* zgrep.in (usage): Document which grep options are not supported.
Problem reported by Liron Paryente in
<http://lists.gnu.org/archive/html/bug-grep/2013-06/msg00005.html>.
tests: zgrep-signal race condition fix

* tests/zgrep-signal: Check that Perl supports dup2.
(exec_with_SIGPIPE_SIGDFL): Remove.
(write_to_dangling_pipe): Simplify by moving more of it into Perl.
Fix race condition, where subcommand writes to a pipe before the ":"
command exits.  Problem reported by Thorsten Glaser in
<http://lists.gnu.org/archive/html/bug-gzip/2013-06/msg00028.html>.