Update .gitignore for 'make check' built files
configure: Enable usual SSSD_WARNINGS automatically

Instead of limiting these to Fedora based sssd developers
via the bashrc_sssd script, enable them automatically in
configure if supported by the compiler.
configure: Add strict compilation mode with --enable-strict
util: Fix const cast failures when building with -Werror

When building with -Werror 'make check' fails with many
errors like:

../src/tests/cmocka/test_authtok.c: In function ‘test_sss_authtok_password’:
../src/tests/cmocka/test_authtok.c:98:48: error: cast discards ‘__attribute__((const))’ qualifier from pointer target type [-Werror=cast-qual]

Make sss_authtok_set() @data argument const, and fix its documentation