dos: add makefile for DOS libfb exports

- added src/rtlib/dos/libfbexp.mk to help make symb_reg.txt
- usage from top-level is 'make -f src/rtlib/dos/libfbexp.mk'

(cherry picked from commit 012f00a)

Author: jayrm

src/rtlib/dos/libfbexp.mk

@@ -0,0 +1,57 @@
+# rebuild symb_reg.txt, needed by sys_dylib.c
+# to build the export table for DOS dylib support
+#
+# expects standalone version
+#
+# running this makefile will touch sys_dylib.c
+# which will cause libfb to be made next time
+# fbc's top-level makefile is run.  Of course,
+# that will cause libfb to be newer, again;
+# which would trigger the build in this makefile
+# if this makefile were to be run again.
+#
+# Currently, there is a cyclic dependency, between
+# fbc's top-level makefile and this makefile that
+# has not been solved out.  But that's OK.  Intent
+# is that it would simply be used from a higher 
+# level script once during build.
+#
+# From top-level fbc directory:
+#    make TARGET_OS=dos
+#    make -f src/rtlib/dos/libexp.mk
+#    make TARGET_OS=dos
+#
+# Also, check for changes in 'symb_reg.txt' and commit
+# changes to repository. 
+
+FBC := fbc-dos.exe
+DXE3GEN := dxe3gen
+CP := cp
+RM := rm -f
+TOUCH := touch
+
+SRCDIR := ./src/rtlib/dos/
+LIBDIR := ./lib/dos/
+BINDIR := ./bin/dos/
+
+.phony: all
+all: $(SRCDIR)symb_reg.txt                
+
+$(SRCDIR)symb_reg.txt: $(LIBDIR)libfblst.txt $(BINDIR)maksymbr.exe
+	$(BINDIR)maksymbr.exe -o $@ $<
+	$(TOUCH) $(SRCDIR)sys_dylib.c
+
+$(BINDIR)maksymbr.exe: $(SRCDIR)maksymbr.bas
+	$(FBC) $< -exx -x $@ 
+
+$(LIBDIR)libfblst.txt: $(LIBDIR)libfb.dxe 
+	$(DXE3GEN) --show-exp $< > $@
+
+$(LIBDIR)libfb.dxe: $(LIBDIR)libfb.a 
+	$(DXE3GEN) -o $@ --whole-archive -U $<
+
+.phony: clean
+clean:
+	$(RM) $(LIBDIR)libfb.dxe
+	$(RM) $(LIBDIR)libfblst.txt
+	$(RM) $(BINDIR)maksymbr.exe