build: Fix missing eq function in version computation

Author: ptrus

common.mk

@@ -48,6 +48,9 @@ GIT_COMMIT_EXACT_TAG := $(shell \
 # Go binary to use for all Go commands.
 export OASIS_GO ?= go
 
+# String equality check function.
+eq = $(and $(findstring $(1),$(2)),$(findstring $(2),$(1)))
+
 # Go command prefix to use in all Go commands.
 GO := env -u GOPATH $(OASIS_GO)