core: Speed up compilation

Author: kairyu

common.mk

@@ -81,7 +81,8 @@ ifdef KEYMAP_SECTION_ENABLE
 endif
 
 # Version string
-OPT_DEFS += -DVERSION=$(shell (git describe --always --dirty || echo 'unknown') 2> /dev/null)
+VERSION := $(shell (git describe --always --dirty || echo 'unknown') 2> /dev/null)
+OPT_DEFS += -DVERSION=$(VERSION)
 
 
 # Search Path