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>

net/pdns-recursor/patches/300-boost-dont-search-host-dirs.patch

@@ -0,0 +1,32 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Eneas U de Queiroz <cotequeiroz@gmail.com>
+Date: Tue, 9 Mar 2021 16:56:13 -0300
+Subject: Openwrt: don't search for boost libs in host dirs
+
+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 patch removes 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>
+
+--- a/m4/boost.m4
++++ b/m4/boost.m4
+@@ -467,8 +467,7 @@ for boost_rtopt_ in $boost_rtopt '' -d;
+     boost_tmp_lib=$with_boost
+     test x"$with_boost" = x && boost_tmp_lib=${boost_cv_inc_path%/include}
+     for boost_ldpath in "$boost_tmp_lib/lib" '' \
+-             /opt/local/lib* /usr/local/lib* /opt/lib* /usr/lib* \
+-             "$with_boost" C:/Boost/lib /lib*
++             "$with_boost"
+     do
+       # Don't waste time with directories that don't exist.
+       if test x"$boost_ldpath" != x && test ! -e "$boost_ldpath"; then