- allow to add additional CFLAGS to build shared libraries

--- makefile.in.orig	2021-09-03 17:45:35 UTC
+++ makefile.in
@@ -3,7 +3,7 @@
 
 SHELL=@SHELL@
 CC=@CC@
-CFLAGS=@CFLAGS@ @MORECFLAGS@
+CFLAGS=@CFLAGS@ @MORECFLAGS@ ${FREEBSD_EXTRA_CFLAGS}
 SAFECFLAGS=@CFLAGS@
 LDFLAGS=@LDFLAGS@ @LIBS@
 THREADLIB=@threadlib@