pdns-recursor: don't search for boost libs in host

pdns-recursor: don't search for boost libs in host

While searching for the boost_system library in boost.m4, configure
tries to find boost_system-mt before boost_system.  The presence of
boost_system-mt in the staging dir depends on
CONFIG_boost-use-name-tags.  If it is not defined (default), and there
is a boost_system-mt library in the host system, it will be used, and
the build will fail.

This adds a patch to remove the host paths from the search loop,
preserving the rest of the detection logic.

Alternatively, boost_cv_lib_context_LIBS could be used to avoid library
detection code entirely, but then the mt- variant would never be used.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>