build: allow CFLAGS_DEFAULT for libs

For reference, see the "all:" target on line 26.

This should have been part of:
    2023-04-08 build: add LIBS variable
    ef14838
and:
    2018-06-28 build: add ability to generate top-level flag files
    3495923

Makefile

@@ -65,6 +65,7 @@ toplevel:	apisupport-config.h cflags-filter.sh	\
 # For "loop-back" building of a subdirectory
 .PHONY:	buildsubdir
 buildsubdir: toplevel
+	export CFLAGS="$${CFLAGS:-${CFLAGS_DEFAULT}}";	\
 	. ./posix-flags.sh;				\
 	. ./cpusupport-config.h;			\
 	. ./cflags-filter.sh;				\
@@ -75,6 +76,7 @@ buildsubdir: toplevel
 # For "loop-back" building of libraries
 .PHONY:	libs
 libs: apisupport-config.h cflags-filter.sh cpusupport-config.h posix-flags.sh
+	export CFLAGS="$${CFLAGS:-${CFLAGS_DEFAULT}}";	\
 	. ./posix-flags.sh;				\
 	. ./cpusupport-config.h;			\
 	. ./cflags-filter.sh;				\