build: add ability to generate top-level flag files

In particular,
- add HAVE_BUILD_FLAGS
- check for HAVE_BUILD_FLAGS in generated Makefiles, and if it isn't present,
  call the top-level Makefile's buildsubdir
- adjust handling of "all_extra:" for the modified Makefile.prog

Makefile

@@ -26,10 +26,18 @@ all:	cpusupport-config.h posix-flags.sh
 	export CFLAGS="$${CFLAGS:-${CFLAGS_DEFAULT}}";	\
 	. ./posix-flags.sh;				\
 	. ./cpusupport-config.h;			\
+	export HAVE_BUILD_FLAGS=1;			\
 	for D in ${PROGS} ${TESTS}; do			\
 		( cd $${D} && ${MAKE} all ) || exit 2;	\
 	done
 
+# For "loop-back" building of a subdirectory
+buildsubdir: cpusupport-config.h posix-flags.sh
+	. ./cpusupport-config.h; \
+	. ./posix-flags.sh; \
+	export HAVE_BUILD_FLAGS=1; \
+	cd ${BUILD_SUBDIR} && ${MAKE} ${BUILD_TARGET}
+
 posix-flags.sh:
 	if [ -d ${LIBCPERCIVA_DIR}/POSIX/ ]; then			\
 		export CC="${CC}";					\

Makefile.BSD

@@ -67,8 +67,8 @@ ${D}/Makefile::
 		printf "\n" >> $@;					\
 		( cd ${D} && awk '/all_extra:/, /^$$/' Makefile.BSD |	\
 		    awk '$$1' ) >> $@;					\
-		( cd ${D} && awk					\
-		    '/all:\t${PROG}/ {print $$0 " all_extra";next}{print}' \
+		( cd ${D} && sed -e					\
+		    's/$${MAKE} $${PROG}/$${MAKE} $${PROG} all_extra/'	\
 		    Makefile > Makefile.new );				\
 		( cd ${D} && mv Makefile.new Makefile );		\
 	fi

Makefile.prog

@@ -1,5 +1,12 @@
 
-all:	${PROG}
+all:
+	if [ -z "$${HAVE_BUILD_FLAGS}" ]; then \
+		cd ${SUBDIR_DEPTH}; \
+		${MAKE} BUILD_SUBDIR=${RELATIVE_DIR} \
+		    BUILD_TARGET=${PROG} buildsubdir; \
+	else \
+		${MAKE} ${PROG}; \
+	fi
 
 install:${PROG}
 	mkdir -p ${BINDIR}