build: fix exporting for make 3.82

build: fix exporting for make 3.82

GNU make 3.82 apparently has a quirk where exporting an undefined
variable prevents its value from subsequently being updated. This
situation can arise due to our adding of -rR to MAKEFLAGS, which takes
effect also on make simply re-invoking itself. Once these flags are in
effect, CC (in particular) is empty (undefined), and would be defined
only via Config.mk including StdGNU.mk or alike. With the quirk, CC
remains empty, yet with an empty CC the compiler minimum version check
fails, breaking the build.

Move the exporting of the various tool stack component variables past
where they gain their (final) values.

See also be63d9d47f57 ("build: tweak variable exporting for make 3.82").

Fixes: 15a0578ca4b0 ("build: shuffle main Makefile")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>