Shared libraries built with sanitizers are a bit inconvenient to use
because they require that any application linking to them is run with
libasan preloaded using LD_PRELOAD. This limitation makes the
sanitizer support less useful because applications will refuse to
start unless there is a special environment variable set.

Let's turn the --enable-address-sanitizer configure flag into
--with-address-sanitizer=yes|no|exec so that is possible to enable
asan only for executables.