When --enable-src is given, the programs in src directory will be
built. If --disable-src is given, those programs will not be built. If
none of them are given, --enable-src is assumed.
To distinguish the to-be-installed programs and non-installable
example source code, the former programs, spdycat, spdydyd and shrpx,
were moved to src directory. spdynative was removed from Makefile
because it does not appeal to any users much.