This is a change introduced in 5.10.0-2ubuntu2 with the changelog:

xfsprogs upstream has regressed config.guess, so use
dh_update_autotools_config.

The 5.10.0 tarball has a config.guess that breaks builds on RISC-V:
...
UNAME_MACHINE = riscv64
UNAME_RELEASE = 5.0.0+
UNAME_SYSTEM  = Linux
UNAME_VERSION = #2 SMP Sat Mar 9 22:34:53 UTC 2019
configure: error: cannot guess build type; you must specify one
make[1]: *** [Makefile:131: include/builddefs] Error 1
...

Reported-by: Steve Langasek <redacted>
Signed-off-by: Bastian Germann <redacted>
---
 debian/changelog | 7 +++++++
 debian/rules     | 1 +
 2 files changed, 8 insertions(+)

diff --git a/debian/changelog b/debian/changelog
index 5421aed6..679fbf03 100644
--- a/debian/changelog
+++ b/debian/changelog

@@ -1,3 +1,10 @@
+xfsprogs (5.11.0-rc0-1) experimental; urgency=medium
+
+  [ Steve Langasek ]
+  * Regenerate config.guess using debhelper
+
+ -- Bastian Germann <bastiangermann@fishpost.de>  Sat, 20 Feb 2021 11:57:31 +0100
+
 xfsprogs (5.10.0-3) unstable; urgency=medium
 
   * Drop unused dh-python from Build-Depends (Closes: #981361)

diff --git a/debian/rules b/debian/rules
index c6ca5491..fe9a1c3a 100755
--- a/debian/rules
+++ b/debian/rules

@@ -43,6 +43,7 @@ config: .census
 	@echo "== dpkg-buildpackage: configure" 1>&2
 	$(checkdir)
 	AUTOHEADER=/bin/true dh_autoreconf
+	dh_update_autotools_config
 	$(options) $(MAKE) $(PMAKEFLAGS) include/platform_defs.h
 	touch .census
 

-- 
2.30.1