M4 macro for autoconf.

AC_LIB_VLIB(min_version)

example:
AC_LIB_VLIB(1.4.1.2)

It will look for v-config and try to compile
and link a sample program. If it fails it will
try again using extra CXXFLAGS and LDFLAGS.
If it succeeds, those flags will be appended.

$VLIB_CONFIG will point to v-config after the test

vlib_static will point to the static archive

