fnmatch: merge from glibc + proposal This merges the change proposed by Adhemerval Zanella in: https://sourceware.org/pipermail/libc-alpha/2020-December/121212.html which fixes a Gnulib bug that led to a failed assert. * lib/fnmatch_loop.c (EXT): Use signed level, not unsigned, and check that it stays nonnegative. Use __flexarr instead of FLEXIBLE_ARRAY_MEMBER, to port better to glibc. * tests/test-fnmatch.c (main): New test cases, taken from glibc.