Primarily, this Fixes #113.

Some minor formatting changes were made where it made the quoted logic
easier to read/follow.

There were also a couple of other improvements that naturally flowed from
the changes in quotation:

Instead of using AC_CHECK_HEADERS() per header, they have now been
coalesced into one call where appropriate.

Adds explicit usage of AC_INCLUDES_DEFAULT as the includes argument to
AC_CHECK_HEADERS(). Without this, some platforms/compilers will complain
that the header was found, but not useable by the compiler. This will
happen on e.g. Solaris where certain  header files have to be included to
pull in the necessary defines. This is achieved through the use
AC_INCLUDES_DEFAULT.

Removes variables that were either not used or only set for use with
AC_SUBST(); HOST_IS_WINDOWS, USE_OPENSSL, PCAP and HAVE_PCAP.
AC_SUBSTS() will happily accept both the name and the value as two
separate arguments - no need to set a variable just for that.

Adds AC_MSG_RESULT() for the AC_MSG_CHECKING() message for user specified
OpenSSL directory. Without this, the configure script would output the
next check on the same line.