Shorten compilation times

Now fish shell stores version is a small file called by other files.
This means that a slight change which modifies one file won't cause
many of files to recompile.

The compilation unit is intentionally small, this is by design. The
smaller it is, the faster it will recompile, and it will be compiled
a lot.

fish_version.cpp

@@ -0,0 +1,14 @@
+/** \file fish_version.c Fish version receiver.
+
+  This file has a specific purpose of shortening compilation times when
+  the only change is different `git describe` version.
+*/
+
+#include "fish_version.h"
+
+/**
+ * Return fish shell version.
+ */
+const char *get_fish_version() {
+    return FISH_BUILD_VERSION;
+}