two more new warning/errors in coreutils/gnulib tests

[Jim Meyering]

In case someone else finds time to address these new warnings:

test-sigprocmask.c: In function 'main':
test-sigprocmask.c:66:10: error: ignoring return value of 'system', declared 
with attribute warn_unused_result [-Werror=unused-result]
cc1: all warnings being treated as errors

make[8]: *** [test-sigprocmask.o] Error 1
make[8]: *** Waiting for unfinished jobs....
  CC       test-strftime.o
test-snprintf.c: In function 'main':
test-snprintf.c:68:9: error: declaration of 'retval' shadows a previous local 
[-Werror=shadow]
test-snprintf.c:37:7: error: shadowed declaration is here [-Werror=shadow]
cc1: all warnings being treated as errors