package/apg: add optional dependency to libxcrypt

package/apg: add optional dependency to libxcrypt

When glibc was bumped to version 2.39 in commit
b5680f53d60acf8ff6010082f873438a39bd5d97 it removed the deprecated
libcrypt support.

As glibc's libcrypt was providing apg's libcrypt dependency this broke
the apg build using glibc version 2.39.

To fix this select the libxcrypt dependency to apg when using a glibc
toolchain and add the dependency if selected.

Fixes:
http://autobuild.buildroot.net/results/3df84ae300bb648492cffc21e4481fc5abf02aeb/

Signed-off-by: Bernd Kuhls <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>