Use autoconf to determine whether system has POSIX signals,

instead of relying on port's os.h to tell us.  (Needed for HPUX
where system major version is not enough info.)
configure unsets USE_TK if X libraries not found.
doc/Makefile uses gzcat or zcat as found by autoconf.

src/include/port/unixware.h

@@ -1,6 +1,5 @@
 #define USE_POSIX_TIME
 #define NO_EMPTY_STMTS
-#define USE_POSIX_SIGNALS
 #define SYSV_DIRENT
 
 #define HAS_TEST_AND_SET