m4: crypt_xpg6.m4 - Define _DEFAULT_SOURCE for current glibc

m4: crypt_xpg6.m4 - Define _DEFAULT_SOURCE for current glibc

Current glibc no longer implements the CRYPT extension, so it does not
declare crypt in <unistd.h> in strict standard modes.  The check
defines _XOPEN_SOURCE, which enables one of these modes.  Defining
_DEFAULT_SOURCE as well again makes available the crypt function
prototype.

This avoids a configure check result change with compilers which do
not support implicit function declarations.