--- a/build/apr_rules.mk.in
+++ b/build/apr_rules.mk.in
@@ -42,10 +42,10 @@
 # compilation and linking flags that are supposed to be set only by the user.
 # configure adds to them for tests, but we restore them at the end.
 #
-CFLAGS=@CFLAGS@
-CPPFLAGS=@CPPFLAGS@
-LDFLAGS=@LDFLAGS@
-LIBS=@LIBS@
+CFLAGS?=@CFLAGS@
+CPPFLAGS?=@CPPFLAGS@
+LDFLAGS?=@LDFLAGS@
+LIBS?=@LIBS@
 DEFS=@DEFS@
 
 # anything added to the standard flags by configure is moved to EXTRA_*
