Document --without-openssl and --without-zlib.

INSTALL

@@ -1,4 +1,3 @@
-1. Prerequisites
 ----------------
 
 A C compiler.  Any C89 or better compiler should work.  Where supported,
@@ -221,6 +220,11 @@ libraries are installed.
 
 --with-ssl-engine enables Libre/OpenSSL's (hardware) ENGINE support
 
+--without-openssl builds without using OpenSSL.  Only a subset of ciphers
+and algorithms are supported in this configuration.
+
+--without-zlib builds without zlib.  This disables the Compression option.
+
 --with-4in6 Check for IPv4 in IPv6 mapped addresses and convert them to
 real (AF_INET) IPv4 addresses. Works around some quirks on Linux.