build: Update ax_boost_mase.m4 to the latest serial

This change fixes boost detection for arch armv7l.

build-aux/m4/ax_boost_base.m4

@@ -33,7 +33,7 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 47
+#serial 48
 
 # example boost program (need to pass version)
 m4_define([_AX_BOOST_BASE_PROGRAM],
@@ -123,6 +123,7 @@ AC_DEFUN([_AX_BOOST_BASE_RUNDETECT],[
     dnl are almost assuredly the ones desired.
     AS_CASE([${host_cpu}],
       [i?86],[multiarch_libsubdir="lib/i386-${host_os}"],
+      [armv7l],[multiarch_libsubdir="lib/arm-${host_os}"],
       [multiarch_libsubdir="lib/${host_cpu}-${host_os}"]
     )