[XEN PATCH v3 1/2] build: evaluate XEN_BUILD_* and XEN_DOMAIN immediately

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

With GNU make 4.4, the number of execution of the command present in
these $(shell ) increased greatly. This is probably because as of make
4.4, exported variable are also added to the environment of $(shell )
construct.

Also, `make -d` shows a lot of these:
    Makefile:15: not recursively expanding XEN_BUILD_DATE to export to shell 
function
    Makefile:16: not recursively expanding XEN_BUILD_TIME to export to shell 
function
    Makefile:17: not recursively expanding XEN_BUILD_HOST to export to shell 
function
    Makefile:14: not recursively expanding XEN_DOMAIN to export to shell 
function

So to avoid having these command been run more than necessary, we
will replace ?= by an equivalent but with immediate expansion.

Reported-by: Jason Andryuk <jandryuk@xxxxxxxxx>
Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---

Notes:
    v3:
    - replace evaluation on first use construct by immediate expansion which
      isn't likely to break and is clearer.

 xen/Makefile | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/xen/Makefile b/xen/Makefile
index e8aa663781..c1738dbbde 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -11,10 +11,18 @@ export XEN_FULLVERSION   = 
$(XEN_VERSION).$(XEN_SUBVERSION)$(XEN_EXTRAVERSION)
 -include xen-version
 
 export XEN_WHOAMI      ?= $(USER)
-export XEN_DOMAIN      ?= $(shell ([ -x /bin/dnsdomainname ] && 
/bin/dnsdomainname) || ([ -x /bin/domainname ] && /bin/domainname || echo 
[unknown]))
-export XEN_BUILD_DATE  ?= $(shell LC_ALL=C date)
-export XEN_BUILD_TIME  ?= $(shell LC_ALL=C date +%T)
-export XEN_BUILD_HOST  ?= $(shell hostname)
+ifeq ($(origin XEN_DOMAIN), undefined)
+export XEN_DOMAIN      := $(shell ([ -x /bin/dnsdomainname ] && 
/bin/dnsdomainname) || ([ -x /bin/domainname ] && /bin/domainname || echo 
[unknown]))
+endif
+ifeq ($(origin XEN_BUILD_DATE), undefined)
+export XEN_BUILD_DATE  := $(shell LC_ALL=C date)
+endif
+ifeq ($(origin XEN_BUILD_TIME), undefined)
+export XEN_BUILD_TIME  := $(shell LC_ALL=C date +%T)
+endif
+ifeq ($(origin XEN_BUILD_HOST), undefined)
+export XEN_BUILD_HOST  := $(shell hostname)
+endif
 
 # Best effort attempt to find a python interpreter, defaulting to Python 3 if
 # available.  Fall back to just `python` if `which` is nowhere to be found.
-- 
Anthony PERARD