Makefile.am update

Update Makefile.am in a few ways.  Let GNU Emacs choose the format of
multiline assignments.  Avoid gratuitous use of scrypt_* automake
variables.  Include all the files in the "make dist" tarball, e.g.,
include *.h by adding them to _SOURCES, as discussed in the automake
info pages.  Avoid stepping on CFLAGS which is reserved for the user,
instead use AM_...
configure.ac standardize

Add standard constructs and comments and do things in standard ways
and in the standard order.
configure.ac detect sse2