build: fix detection of boost libs on multi-arch distros

Fixes #3219.

src/m4/ax_boost_base.m4

@@ -33,7 +33,7 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 21
+#serial 22
 
 AC_DEFUN([AX_BOOST_BASE],
 [
@@ -97,6 +97,12 @@ if test "x$want_boost" = "xyes"; then
         ;;
     esac
 
+    dnl allow for real multi-arch paths e.g. /usr/lib/x86_64-linux-gnu. Give
+    dnl them priority over the other paths since, if libs are found there, they
+    dnl are almost assuredly the ones desired.
+    AC_REQUIRE([AC_CANONICAL_HOST])
+    libsubdirs="lib/${host_cpu}-${host_os} $libsubdirs"
+
     dnl first we check the system location for boost libraries
     dnl this location ist chosen if boost libraries are installed with the --layout=system option
     dnl or if you install boost with RPM