Customized make file inclusion

[David Holmes]

Presently the build system contains some files, and directives involving 
those files, that supports the building of the Java SE Embedded product. 
What I would like to do is remove direct support for this and replace it 
with a more general mechanism that allows for custom make files to be 
included from an arbitrary location - which then allows for the removal 
of the embedded specific files.

Initially I've modeled this on some other conditional mechanisms by 
defining the default location for non-OpenJDK builds, checking its 
existence and then using that as a guard for the individual files.

http://cr.openjdk.java.net/~dholmes/custom-make/webrev/

! ifneq ($(OPENJDK),true)
!   CUSTOM_MAKE_DIR_REL=closed
! else
!   CUSTOM_MAKE_DIR_REL=NO_SUCH_PATH
! endif
! CUSTOM_MAKE_DIR=$(BUILDDIR)/$(CUSTOM_MAKE_DIR_REL)

+ # Use this variable to guard inclusion of the custom files
+ HAS_CUSTOM_MAKE := $(shell if [ -d $(CUSTOM_MAKE_DIR) ]; then echo 1; 
else echo 0; fi)

...

+ ifeq ($(HAS_CUSTOM_MAKE),1)
+   include $(CUSTOM_MAKE_DIR)/Defs.gmk
+ endif

Initially there are only two hooks for these custom files:

1. At the end common/Defs.gmk (as it gets included by all the Makefiles)
2. In the top-level JDK Makefile, including a custom Release.gmk

Naturally these map to the existing uses of the *-embedded.gmk files.

It then occurred to me that if someone wanted to add an additional hook 
somewhere else, that it might be best to check for the existence of the 
actual file rather than a root directory for all such files. That led me 
to consider a function custom_include(<filename>) to hide the existence 
logic. But I wasn't sure if a function could contain an include 
directive and upon checking on that I discovered a much simpler 
approach: the -include directive. This is like the include directive but 
does not trigger an error if the file does not exist. So we would simply 
have this in Defs.gmk:

  ifneq ($(OPENJDK),true)
    CUSTOM_MAKE_DIR_REL=closed
  else
    CUSTOM_MAKE_DIR_REL=NO_SUCH_PATH
  endif
  CUSTOM_MAKE_DIR=$(BUILDDIR)/$(CUSTOM_MAKE_DIR_REL)

and then

-include $(CUSTOM_MAKE_DIR)/<path-to-file.gmk>

wherever we needed it (again initially only the two locations I mentioned).

I'd like to hear people's opinions on this.

Thanks,
David