Fix build targets in Makefile.prebuild
Fix build targets in Makefile.prebuild

- config.h was used as target even when it wasn't generated.
  This only worked because the 'dummy' target always triggers
  a full rebuild.
  It is however better to specify the exact target that is to
  be rebuilt do avoid confusion.
- Explicitly mark 'dummy' as a 'phony' target.

Signed-off-by: Egbert Eich <d0f3508a0b242623257f3971a925c8211f91345e@suse.com>

xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS