configure.ac: fix detection of secure_getenv

configure.ac: fix detection of secure_getenv

lib/defines.h was looking for HAVE_SECURE_GETENV instead of of
HAS_SECURE_GETENV as defined in configure. This resulted in
shadow_getenv always being defined to getenv.

AC_CHECK_FUNC is linker test; it does not check for declarations.
Replace this with AC_CHECK_DECLS/HAVE_DECL_SECURE_GETENV.

Fixes: 3d921155e0a7 (2019-03-31; "gettime: Use secure_getenv over getenv.")
Signed-off-by: Mike Gilbert <floppym@gentoo.org>