meson: Use configure_file when version-tag is specified

vcs_tag() is slow. When the version-tag meson option is set,
we can use configure_file() directly to speed up incremental
builds.

Before (with version-tag set to v247):

```
‣ Running build script...
[1/418] Generating version.h with a custom command

real    0m0.521s
user    0m0.229s
sys     0m0.067s
```

After (with version-tag set to v247):

```
‣ Running build script...
ninja: no work to do.

real    0m0.094s
user    0m0.048s
sys     0m0.022s
```