Update illumos support to the modern era (#24)

* Update Solaris / Illumos support

Old versions of Solaris did not have vasprintf, so darkhttpd defined one
gated behind an ifdef. Oracle Solaris 10 has had vasprintf since 2011.
Oracle Solaris 11 has had it since release. illumos (which also reports
as `__sun`) also has it in all current incarnations. As a result, this
ifdef'd block creates compiler errors due to a second definition of the
function. This commit removes the block.

This commit also adds `-lsendfile` to the Makefile for systems that
report as `SunOS` in `uname` (Solaris and Illumos), which is necessary
to link successfully in current day.

* Comment on manually specifying CC in readme

Some systems, including versions of illumos I use, do not have a `cc`
alias to the system C compiler. Arguably this is a flaw in the
distribution, but as a user, it's perhaps helpful to be reminded that
this is an option.

Makefile

@@ -1,6 +1,6 @@
 CC?=cc
 CFLAGS?=-O
-LIBS=`[ \`uname\` = "SunOS" ] && echo -lsocket -lnsl`
+LIBS=`[ \`uname\` = "SunOS" ] && echo -lsocket -lnsl -lsendfile`
 
 all: darkhttpd