allow to build with older sed

sed's -E option may not be supported by older distros. As there's no
point using sed here at all, use just shell mechanisms to establish the
variable values, starting from the stem instead of the full target.
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Makefile

@@ -256,8 +256,7 @@ GENERATED_FILES += $(KEYCODEMAP_FILES)
 
 ui/input-keymap-%.c: $(KEYCODEMAP_GEN) $(KEYCODEMAP_CSV) $(SRC_PATH)/ui/Makefile.objs
 	$(call quiet-command,\
-	    src=$$(echo $@ | sed -E -e "s,^ui/input-keymap-(.+)-to-(.+)\.c$$,\1,") && \
-	    dst=$$(echo $@ | sed -E -e "s,^ui/input-keymap-(.+)-to-(.+)\.c$$,\2,") && \
+	    stem=$* && src=$${stem%-to-*} dst=$${stem#*-to-} && \
 	    test -e $(KEYCODEMAP_GEN) && \
 	    $(PYTHON) $(KEYCODEMAP_GEN) \
 	          --lang glib2 \