Makefile: fix up parallel building under MSYS+MinGW

This patch enables parallel building of QEMU in MSYS+MinGW environment.
Currently an attempt to build QEMU in parallel fails on generation of
version.lo (and version.o too).

The cause of the failure is that when listing prerequisites "Makefile"
references "config-host.h" by absolute path in some rules and by relative
path in others. Make cannot figure out that these references points to the
same file which leads to the race: the generation of "version.*" which
requires "$(BUILD_DIR)/config-host.h" is launched in parallel with the
generation of "config-host.h" needed by other "Makefile" targets.

This patch removes "$(BUILD_DIR)/" prefix from corresponding prerequisite
of "version.*". There is no other prerequisites "$(BUILD_DIR)/config-host.h"
found.

Also note that not every version of MSYS is able to build QEMU in parallel,
see: "http://sourceforge.net/p/mingw/bugs/1950/". The suggested version is
1.0.17.

Signed-off-by: Vasily Efimov <real@ispras.ru>
Message-Id: <1424264377-5992-1-git-send-email-real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
