Merge bitcoin#19152: build: improve build OS configure output

0fef60c build: improved output of configure for build OS (sachinkm77)

Pull request description:

  The purpose of this fix is to improve output of the configure script by providing the build OS. This is done by leveraging the build_os set by the script config.sub / config.guess. bitcoin#18966

ACKs for top commit:
  fanquake:
    ACK 0fef60c - thanks for following up.

Tree-SHA512: b9f49df901a9d37eb16c67c063bb3611602a84391aa54d097a52b740f474c2785c24bf405522d15d724fde25070d354bf20b885add2ee4405a71cbe9ebab5ff3

configure.ac

@@ -1871,7 +1871,7 @@ echo "  gprof enabled       = $enable_gprof"
 echo "  werror              = $enable_werror"
 echo
 echo "  target os           = $TARGET_OS"
-echo "  build os            = $BUILD_OS"
+echo "  build os            = $build_os"
 echo
 echo "  CC                  = $CC"
 echo "  CFLAGS              = $PTHREAD_CFLAGS $CFLAGS"