Make prep lib

This makefile directive launches the following commands:

 $(REBOL) $T/make-headers.r
 $(REBOL) $T/make-boot.r $(OS_ID)
 $(REBOL) $T/make-os-ext.r # ok, but not always
 $(REBOL) $T/make-reb-lib.r
-rm -f $(OBJS)

substituting the variable values it means:

 ./r3-make  ../src/tools/make-headers.r
 ./r3-make  ../src/tools/make-boot.r $(OS_ID)
 ./r3-make  ../src/tools/make-os-ext.r # ok, but not always
 ./r3-make  ../src/tools/make-reb-lib.r
-rm -f $(OBJS)

$(OS_ID) is the operating system identifier, it is 0.3.1 for windows.

$(OBJS) is a list of file inside the obj/ folder, click on link for further information.

1. The first line launches [[make-headers.r]] script
2. launches [[make-boot.r]] 0.3.1
3. launches [[make-os-ext.r]]
4. launches [[make-reb-lib.r]]

the line before the last command impose to the makefile to ignore output error that command line, so tha last command line is just:

rm -f $(OBJS)

It removes all obj/* files. (my perplexity is: do we need to remove obj/* files? One of the greatest feature of makefile is to check and rebuild file only if necessary. Deleting them, we recompile the same files many times. @angerangel)