diff --git a/Makefile b/Makefile
index ce97c5efa..61b226bac 100644
--- a/Makefile
+++ b/Makefile
@@ -134,6 +134,20 @@ ifneq ($(OS),bsd)
   CWFLAGS+=-Wno-error=unused-result
 endif
 
+# glibc 2.24 deprecates readdir_r; iff glibc >=2.24,
+# don't upgrade 'deprecated declarations' warnings to errors
+# dependency: `getconf`, which comes w/glibc
+GLIBC := $(lastword $(shell getconf GNU_LIBC_VERSION))
+# dependency: none, uses make's native functions
+GLIBC_MAJ := $(word 1, $(subst ., ,$(GLIBC))) 
+GLIBC_MIN := $(word 2, $(subst ., ,$(GLIBC)))
+# dependency: `expr` shell built-in
+GLIBC_GE_2_24 := $(shell expr $(GLIBC_MAJ) ">" 2 "|" \
+        $(GLIBC_MAJ) "=" 2 "&" $(GLIBC_MIN) ">=" 24)
+ifeq (1,$(GLIBC_GE_2_24))
+  CWFLAGS+=-Wno-error=deprecated-declarations
+endif
+
 ifdef NO_SILENT_RULES
 %.o: %.c $(CORE)
 	$(CC) -c $(CWFLAGS) $(CFLAGS) -o $@ $<
diff --git a/include/noun/allocate.h b/include/noun/allocate.h
index 7ab6b2139..68de4afb8 100644
--- a/include/noun/allocate.h
+++ b/include/noun/allocate.h
@@ -23,7 +23,7 @@
   **/
     /* u3a_bits: number of bits in word-addressed pointer.  29 == 2GB.
     */
-#     define u3a_bits  29
+#     define u3a_bits  U3_OS_LoomBits
 
     /* u3a_page: number of bits in word-addressed page.  12 == 16Kbyte page.
     */