boost: ignore site config during host build

Without this, the host build picks up the site config in
/usr/share/boost-build/site-config.jam which causes a build failure on
Gentoo where a custom optimization value "none" is used.

Fixes #9152.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

libs/boost/Makefile

@@ -384,7 +384,7 @@ define Host/Compile
 				headers \
 				$(foreach lib,$(BOOST_LIBS), \
 					$(if $(findstring python,$(lib)),,$(if $(CONFIG_boost-host-build-$(lib)),$(lib)))))) ; \
-		./b2 install )
+		./b2 --ignore-site-config install )
 endef
 
 CONFIGURE_PREFIX:=$(PKG_INSTALL_DIR)