configure.ac libcrypto

Migrate -lcrypto from Makefile.am to configure.ac, and actually test
that it provides the desired AES API.
Makefile.am sse2

Use automake conditional for enabling/disabling sse2.  This gets the right
files into "make dist" and is more robust.  Basically, the approved
mechanism.  It also slightly clarifies the sse2 option in configure.ac,
which has to be tweaked to use AM_CONDITIONAL instead of AC_SUBST.
Makefile.am update

Update Makefile.am in a few ways.  Avoid tabs inside variables which
messes up cut-and-paste of generated shell commands in terminal
windows.  Avoid gratuitous use of scrypt_.  Include all the files
in the "make dist" tarball, include *.h by adding them to _SOURCES,
which is the approved way per the automake info pages.  Avoid stepping
on CFLAGS which is reserved for the user, instead use AM_...