Fix build for mingw32 on windows ($@ in macro)

Fix build for mingw32 on windows ($@ in macro)

Make using mingw32 on windows does not preserve $@ in macros
when they are modified using this pattern:
target: macro += something

This behaviour results in an error when QEMU_CFLAGS containing
"-MMD -MP -MT $@" is modified for compilation of source files
which use SDL: $@ will expand to nothing, -MT no longer has
the correct argument (it will take the next one from the command
line) and the build will fail or run with a wrong command line.

The problem is fixed by using a new macro QEMU_DGFLAGS
which is not modified by a target rule.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>