diff -ru clzip-1.12.orig/configure clzip-1.12/configure
--- clzip-1.12.orig/configure	2021-01-01 16:37:55.000000000 +0100
+++ clzip-1.12/configure	2021-01-13 12:42:25.898898082 +0100
@@ -22,7 +22,7 @@
 mandir='$(datarootdir)/man'
 CC=gcc
 CPPFLAGS=
-CFLAGS='-Wall -W -O2'
+CFLAGS ?= '-Wall -W -O2'
 LDFLAGS=
 
 # checking whether we are using GNU C.