Fixes AC_CHECK_HEADER for issue #3

https://github.com/plashchynski/viewssld/issues/3

https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/Present-But-Cannot-Be-Compiled.html

Still not finding dssl/sslcap.h but at least the error message is a little nicer. Here is the new snippet on Ubuntu 12.04.2 LTS:
"
.
.
.
checking openssl/ssl.h usability... yes
checking openssl/ssl.h presence... yes
checking for openssl/ssl.h... yes
checking for dssl/sslcap.h... no
configure: error: libdssl header file not found. https://github.com/downloads/plashchynski/viewssld/libdssl-2.1.1.tar.gz
jlowry@lappie:~/src/c/viewssld$ 
.
.
.
"