Address trivial build issues on certain Linux distribuitions (e.g.
Fedora 20 on which this patch was tested).
Most standard C++ libraries had unistd.h removed in GCC 4.7, so add it
back in explicitly.
Resolve the following ASN.1 build error by specifying the C language
tag.
libtool: compile: unable to infer tagged configuration
libtool: compile: specify a tag with `--tag'
Signed-off-by: Tom Tsou <tom@tsou.cc>