build-sys: fix type detection

build-sys: fix type detection

Before this patch existence of char16_t, char32_t, key_serial_t was checked
with AC_CHECK_DECLS() which doesn't actually work for types. Correct this to
use AC_CHECK_TYPES() instead.

Also, while we are at it, change the check for memfd_create() to use
AC_CHECK_DECLS() instead of AC_CHECK_FUNCS(). This is a better choice, since a
couple of syscalls are defined by glibc but not exported in the header files
(pivot_root() for example), and we hence should probably be more picky with
memfd_create() too, which glibc might decide to expose one day, but not
necessarily in the headers too.