Don't override CFLAGS.
--- configure.orig	2006-01-30 06:12:24.000000000 -0600
+++ configure	2021-11-23 06:09:53.000000000 -0600
@@ -3402,44 +3402,44 @@
 
 case $host_os in
   *bsd*)
-    CFLAGS="-D__FreeBSD__"   # danger! danger! danger!?
+    CFLAGS+=" -D__FreeBSD__"   # danger! danger! danger!?
     PING_HEAD=''
     PING_TAIL='56 1'
     echo "checking for ping once syntax... bsd... $PING host 56 1"
   ;;
   *linux*)
-    CFLAGS="-D__LINUX__"
+    CFLAGS+=" -D__LINUX__"
     PING_HEAD='-c 1'
     PING_TAIL=''
     echo "checking for ping once syntax... linux... $PING -c 1 host"
   ;;
   *solaris*)
-    CFLAGS="-D__SOLARIS__"
+    CFLAGS+=" -D__SOLARIS__"
     LDFLAGS="-lsocket -lnsl"
     PING_HEAD='-s'
     PING_TAIL='56 1'
     echo "checking for ping once syntax... solaris... $PING -s host 56 1"
   ;;
   *osf*)
-    CFLAGS="-D__OSF1__"
+    CFLAGS+=" -D__OSF1__"
     PING_HEAD='-c 1'
     PING_TAIL=''
     echo "checking for ping once syntax... osf1... $PING -c 1 host"
   ;;
   *sysv4*)
-    CFLAGS="-D__OSF1__"
+    CFLAGS+=" -D__OSF1__"
     PING_HEAD='-c 1'
     PING_TAIL=''
     echo "checking for ping once syntax... svr4... $PING -c 1 host"
   ;;
   *aix*)
-    CFLAGS="-D__AIX__"
+    CFLAGS+=" -D__AIX__"
     PING_HEAD='-c 1'
     PING_TAIL=''
     echo "checking for ping once syntax... aix... $PING -c 1 host"
   ;;
   *sunos*)
-    CFLAGS="-D__SUNOS__"
+    CFLAGS+=" -D__SUNOS__"
     PING_HEAD='-c'
     PING_TAIL='56 1'
     echo "checking for ping once syntax... *unknown os*... $PING -c host 56 1"