[master] set the library version for libdhcp++

The libdhcp++ API was changed as part of the refactoring.

When linking new tests to old library it fails with:

/libexec/ld-elf.so.1:
/usr/home/jreed/builder/work/BIND10-cppcheck/20120516153001-FreeBSD8-amd64-GCC/build/src/bin/dhcp4/tests/.libs/dhcp4_unittests:
Undefined symbol "_ZN3isc4dhcp6OptionC1ENS1_8UniverseEtRKSt6vectorIhSaIhEE"

This is:
 isc::dhcp::Option::Option(isc::dhcp::Option::Universe, unsigned short,
std::vector<unsigned char, std::allocator<unsigned char> > const&)

See http://bind10.isc.org/wiki/LibraryVersioning for details.

src/lib/dhcp/Makefile.am

@@ -29,3 +29,4 @@ EXTRA_DIST  = README
 libdhcp___la_CXXFLAGS = $(AM_CXXFLAGS)
 libdhcp___la_CPPFLAGS = $(AM_CPPFLAGS) $(LOG4CPLUS_INCLUDES)
 libdhcp___la_LIBADD   = $(top_builddir)/src/lib/util/libutil.la
+libdhcp___la_LDFLAGS = -no-undefined -version-info 1:0:0