Adjust to recent gnulib changes.

* lib/.cvsignore: Remove stpcpy.h, strndup.h, strnlen.h.
Add string.h, string_.h, unistd_.h, wchar_.h.
* m4/.cvsignore: Add gnulib-common.m4, string_h.m4, wchar.m4.
* src/system.h: Don't include <stpcpy.h>; this is now done by
<string.h>.

m4/.cvsignore

@@ -10,6 +10,7 @@ extensions.m4
 getopt.m4
 gettext.m4
 gnulib-cache.m4
+gnulib-common.m4
 gnulib-comp.m4
 gnulib-tool.m4
 hash.m4
@@ -36,6 +37,7 @@ stdint_h.m4
 stdio-safer.m4
 stpcpy.m4
 strerror.m4
+string_h.m4
 strndup.m4
 strnlen.m4
 strtol.m4
@@ -45,6 +47,7 @@ unistd-safer.m4
 unistd_h.m4
 unlocked-io.m4
 warning.m4
+wchar.m4
 wchar_t.m4
 wctype.m4
 wcwidth.m4