meson: fix version generation from git

run_command() was only run at configure time, meaning the generated
version (that was passed on to the sources via -DYAMBAR_VERSION)
became stale.

Fix by implementing a shell script that generates a header file, and
wrap this in a custom target that is run every time (but the generated
file is only updated when the version changes)

main.c

@@ -22,6 +22,7 @@
 
 #define LOG_MODULE "main"
 #include "log.h"
+#include "version.h"
 
 static volatile sig_atomic_t aborted = 0;