Honor CFLAGS, CPPFLAGS, and LDFLAGS set at configure time.
--- Makefile.in.orig	2002-01-17 15:27:28.000000000 -0600
+++ Makefile.in	2021-08-13 07:07:31.000000000 -0500
@@ -34,8 +34,9 @@
 LIBOBJS = @LIBOBJS@
 
 ## doesn't compile with followimg -ansi option under BSD/OS
-CFLAGS = -g -O3 -Wall -pedantic -ansi
-LDFLAGS =
+CFLAGS = -g -O3 -Wall -pedantic -ansi @CFLAGS@
+CPPFLAGS = -I. @CPPFLAGS@
+LDFLAGS = @LDFLAGS@
 
 prefix = @prefix@
 exec_prefix = @exec_prefix@