--- exim-4.94.org/OS/Makefile-Linux	2020-05-30 22:35:38.000000000 +0200
+++ exim-4.94/OS/Makefile-Linux	2020-11-27 08:08:51.397823884 +0100
@@ -16,7 +16,7 @@ CHMOD_COMMAND=look_for_it
 # The system cc may be gcc or clang; do not force gcc
 CC=cc
 # Preserve CFLAGS and CFLAGS_DYNAMIC from the caller/environment
-CFLAGS ?= -O -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
+CFLAGS ?= -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $(CUSTOM_CFLAGS)
 CFLAGS_DYNAMIC ?= -shared -rdynamic
 
 DBMLIB = -ldb