nsis: use "make DESTDIR=" instead of "make prefix="

The next patch will prevent modifying the prefix on "make install".  Adjust the
creation of the installer.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -1188,16 +1188,16 @@ installer: $(INSTALLER)
 
 INSTDIR=/tmp/qemu-nsis
 
-$(INSTALLER): install-doc $(SRC_PATH)/qemu.nsi
-	$(MAKE) install prefix=${INSTDIR}
+$(INSTALLER): $(SRC_PATH)/qemu.nsi
+	$(MAKE) install DESTDIR=${INSTDIR}
 ifdef SIGNCODE
-	(cd ${INSTDIR}; \
+	(cd ${INSTDIR}/${bindir}; \
          for i in *.exe; do \
            $(SIGNCODE) $${i}; \
          done \
         )
 endif # SIGNCODE
-	(cd ${INSTDIR}; \
+	(cd ${INSTDIR}/${bindir}; \
          for i in qemu-system-*.exe; do \
            arch=$${i%.exe}; \
            arch=$${arch#qemu-system-}; \
@@ -1206,11 +1206,11 @@ endif # SIGNCODE
            echo File \"\$${BINDIR}\\$$i\"; \
            echo SectionEnd; \
          done \
-        ) >${INSTDIR}/system-emulations.nsh
+        ) >${INSTDIR}/${bindir}/system-emulations.nsh
 	makensis $(nsisflags) \
                 $(if $(BUILD_DOCS),-DCONFIG_DOCUMENTATION="y") \
                 $(if $(CONFIG_GTK),-DCONFIG_GTK="y") \
-                -DBINDIR="${INSTDIR}" \
+                -DBINDIR="${INSTDIR}/${bindir}" \
                 $(if $(DLL_PATH),-DDLLDIR="$(DLL_PATH)") \
                 -DSRCDIR="$(SRC_PATH)" \
                 -DOUTFILE="$(INSTALLER)" \