Changes

makefiles: move the inclusion of Makefile.build_config in Makefile.common
 (commit: fab58a9)

[minor] rename CAN_BE_UNCONFIGURED into REQUIRES_CONFIGURATION
 (commit: 472a20d)

build system: skip Makefile.build_config when REQUIRES_CONFIGURATION is empty

Before we would include it if present, but support building without
it. Unless I misunderstand something, this is unnecessarily complex:

- If we want to support building without it, we may as well
  do without it all the time (in particular it makes it easier
  to spot bugs which occurs when it is missing).

- Certain makefile rules (see config.status) work by being added
  as dependencies to Makefile.build_config, and requiring this file
  only when actually needed makes it possible to simplify those.
  (simplifications not included in this commit)

I reviewed all 'clean' rules to check that they do not depend on
variables defined in the configure. On the other hand, the main
makefiles do a lot of conditional tests on those variables,
so --warn-undefined-variables warns quite a bit. (This could be solved
by a Makefile.no_config rule that mirrors Makefile.config with dummy
definitions.)
 (commit: 21f5ad3)

Makefile: no need for REQUIRES_CONFIGURATION check on config.status

Now that Makefile.build_config is only included when configuration is
required, we can remove the REQUIRES_CONFIGURATION logic here.
 (commit: 8d716a1)

move the REQUIRES_CONFIGURATION logic in Makefile.config_if_required

This lets us share the logic between Makefile.common and Makefile.tools.
 (commit: e34e622)

refactor Makefile.config_if_required for easier reuse of variables
 (commit: 3489e00)

ocamltest/Makefile: do not depend on UNIX_OR_WIN32 for 'clean'

Change suggested by David Allsopp.
 (commit: 11c9d92)

[minor] Makefile: move ARCH_SPECIFIC back to the main Makefile

(This is a small, independent readability fix.)

ARCH_SPECIFIC was moved into Makefile.compilerlibs by mistake; it is
not used in Makefile.compilerlibs (in particular the
ARCH_SPECIFIC_ASMCOMP_* variables do not depend on it), and it is used
in the main Makefile.
 (commit: 67f6128)

[minor] stdlib/Makefile: remove redundant clean action

There is already a more precise

    clean::
            rm -f $(CAMLHEADERS)

rule above.

(Change suggested by David Allsopp.)
 (commit: fd789b5)

Changes entry
 (commit: ffbea08)