maint: avoid gnulib-tool deprecation warnings, and build failure

* bootstrap.conf (gnulib_modules): Use getopt-gnu, rather than
obsolete "getopt" module. Use nstrftime in place of strftime.
(avoided_gnulib_modules): Do not avoid the "lock" module. That
would lead to a build failure due to "glthread/lock.h not found".

bootstrap.conf

@@ -17,7 +17,6 @@
 
 avoided_gnulib_modules='
   --avoid=localename
-  --avoid=lock
   --avoid=lock-tests
   --avoid=setlocale
 '
@@ -42,7 +41,7 @@ fdl
 file-type
 filenamecat
 fnmatch-gnu
-getopt
+getopt-gnu
 gettext-h
 gettime
 git-version-gen
@@ -62,6 +61,7 @@ manywarnings
 mbrtowc
 mkstemp
 mktime
+nstrftime
 progname
 propername
 rawmemchr
@@ -74,7 +74,6 @@ stat-macros
 stat-time
 stdint
 strcase
-strftime
 strptime
 strtoimax
 sys_wait