configure.ac: use PKG_CHECK_MODULES and AC_PATH_PROG to find protobuf-c

since we're going to depend on protobuf-c 1.x, we now no longer need the
custom MY_CHECK_LIBPROTOBUF_C macro. switch to just using
PKG_CHECK_MODULES to find the library and AC_PATH_PROG to find the
compiler.
configure.ac: remove redundant m4_include

we don't actually need to explicitly include m4/nmsg_msgmod.m4.  since
it's in the m4/ subdirectory it will automatically be included.
configure.ac: explicitly depend on libnmsg >= 0.9.0
m4/nmsg_msgmod.m4: nmsg msgmod ABI 8 -> 9
Makefile.am: nmsg msgmod ABI 8 -> 9
configure.ac: bump version to 0.17.0
README.md: new
README.md: fix the debian farsightsec link