[libvirt] [PATCH] build: Fix build with old automake

[Jiri Denemark]

On Mon, Mar 17, 2014 at 09:33:11 -0600, Eric Blake wrote:
> On 03/14/2014 12:55 PM, Jiri Denemark wrote:
> > On Fri, Mar 14, 2014 at 07:54:58 -0600, Eric Blake wrote:
> >> On 03/14/2014 04:43 AM, Jiri Denemark wrote:
> >>> Ancient automake (such as from RHEL5) does not provide abs_srcdir and
> >>> abs_builddir variables which are used by a recent commit of mine
> >>> (e562e82).
> >>>
> >>> Signed-off-by: Jiri Denemark <jdenemar at redhat.com>
> >>> ---
> 
> >>> +# old automake does not provide abs_{src,build}dir variables
> >>> +abs_builddir = $(shell pwd)
> >>> +abs_srcdir = $(shell cd $(srcdir) && pwd)
> 
> Hmm, just noticed another thing - with NEWER automake, we are now less
> efficient (calling out to $(shell) overwrites the value that is already
> provided for free without a subprocess by newer automake).  Does it work
> if you use:
> 
> abs_builddir ?= $(shell pwd)

Automake does a fair amount of magic here so abs_builddir = $(shel ...)
does not actually overwrite the old value because there is no old value
:-) Automake just does not put it's own abs_builddir definition in that
case. If I switch to abs_builddir ?= $(shell ...), automake adds it's
own definition so it seems it could work except that it doesn't. With
"=", automake adds the definition above the other variable definition
and namely above libvirt_cpu_la_DEPENDENCIES which uses it:

    abs_builddir = $(shell ...)
    ...
    libvirt_cpu_la_DEPENDENCIES = $(abs_builddir)/...
    ...
    rules

When I switch to "?=", automake apparently does not recognize it as a
variable definition and puts it between the block with variable
definitions and rules. Thus the result is:

    abs_builddir = /some/build/path
    ...
    libvirt_cpu_la_DEPENDENCIES = $(abs_builddir)/...
    ...
    abs_builddir ?= $(shell ...)
    ...
    rules

which of course does not work with old automake which does not add the
first abs_builddir definition there and thus libvirt_cpu_la_DEPENDENCIES
sees $(abs_builddir) empty.

Anyway, making two extra shell commands once per build does not seem
like anything we should really care about so I'd just stick with

    abs_builddir = $(shell ...)

> The $(abs_srcdir) could be inlined with ``, but you are right that the
> use of $(abs_builddir) has to be expanded up front for make to track the
> right file in its dependency.  So ACK to this patch if the switch to ?=
> instead of = works.

It doesn't.

Jirka