diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN
index 2fee5e7e80..8dc54f6378 100755
--- a/GIT-VERSION-GEN
+++ b/GIT-VERSION-GEN
@@ -56,8 +56,16 @@ then
 	GIT_VERSION=$(expr "$VN" : v*'\(.*\)')
 fi
 
-GIT_BUILT_FROM_COMMIT=$(git -C "$SOURCE_DIR" rev-parse -q --verify HEAD 2>/dev/null)
-GIT_DATE=$(git -C "$SOURCE_DIR" show --quiet --format='%as' 2>/dev/null)
+if test -z "$GIT_BUILT_FROM_COMMIT"
+then
+	GIT_BUILT_FROM_COMMIT=$(git -C "$SOURCE_DIR" rev-parse -q --verify HEAD 2>/dev/null)
+fi
+
+if test -z "$GIT_DATE"
+then
+	GIT_DATE=$(git -C "$SOURCE_DIR" show --quiet --format='%as' 2>/dev/null)
+fi
+
 if test -z "$GIT_USER_AGENT"
 then
 	GIT_USER_AGENT="git/$GIT_VERSION"
diff --git a/shared.mak b/shared.mak
index a66f46969e..1a99848a95 100644
--- a/shared.mak
+++ b/shared.mak
@@ -121,6 +121,8 @@ endef
 # absolute path to the root source directory as well as input and output files
 # as arguments, in that order.
 define version_gen
+GIT_BUILT_FROM_COMMIT="$(GIT_BUILT_FROM_COMMIT)" \
+GIT_DATE="$(GIT_DATE)" \
 GIT_USER_AGENT="$(GIT_USER_AGENT)" \
 GIT_VERSION="$(GIT_VERSION_OVERRIDE)" \
 $(SHELL_PATH) "$(1)/GIT-VERSION-GEN" "$(1)" "$(2)" "$(3)"