diff --git a/ChangeLog.d/linux-aarch64-hwcap.txt b/ChangeLog.d/linux-aarch64-hwcap.txt
new file mode 100644
index 0000000..23af878
--- /dev/null
+++ b/ChangeLog.d/linux-aarch64-hwcap.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * On Linux on ARMv8, fix a build error with SHA-256 and SHA-512
+     acceleration detection when the libc headers do not define the
+     corresponding constant. Reported by valord577.
diff --git a/library/sha512.c b/library/sha512.c
index 6011254..6dcea8d 100644
--- a/library/sha512.c
+++ b/library/sha512.c
@@ -102,6 +102,14 @@
 #      if defined(__linux__)
 /* Our preferred method of detection is getauxval() */
 #        include <sys/auxv.h>
+#        if !defined(HWCAP_SHA512)
+/* The same header that declares getauxval() should provide the HWCAP_xxx
+ * constants to analyze its return value. However, the libc may be too
+ * old to have the constant that we need. So if it's missing, assume that
+ * the value is the same one used by the Linux kernel ABI.
+ */
+#          define HWCAP_SHA512 (1 << 21)
+#        endif
 #      endif
 /* Use SIGILL on Unix, and fall back to it on Linux */
 #      include <signal.h>