Add InMon's sFlow Agent library to the build system.

Add InMon's sFlow Agent library to the build system.

The C source and header files added in this commit is covered under the
InMon sFlow license at http://www.inmon.com/technology/sflowlicense.txt

The library requires -Wno-unused to compile without warnings, so this
commit adds that for building the sFlow code only.  Automake can only
change compiler flags on a per-library or per-program basis, so sFlow
is built as a separate library.

The library will be used in upcoming commits.