GNUmakefile: slightly better structure

clumping all `.PHONY: whatever` lines together at the end of
the makefile is a good way to get them out of sync with actual
phony targets.
bsu_osc_build: new configuration variable

the difference between empty and populated `$bsu_osc_build` is::

  --- tests/630-build-success/tape.expected
  +++ tests/650-osc-build-args/tape.expected
  @@ -15,7 +15,7 @@
   sed -ri "/^[ \t]*#/!s/__VERSION__/12.34/" pkg.spec
   command osc -A obs st pkg.spec
   test 1 -eq 1
  -command osc -A obs build
  +command osc -A obs build Arch_Extra x86_64 PKGBUILD
   test 1 -eq 1
   command osc -A obs ci -m "Update to version 12.34"
   rm -rf /tmp/fake-tmp-dir