build: support selecting enabled utils in Meson

Add a new Meson array option, utils, to allow building only selected applets while keeping the default as all.

Gate source inclusion, dependencies, symlink/manpage/script install steps, and test registration by enabled util set, and make applet dispatch in src/main.c compile-time conditional via generated config defines.

Document selective build usage in README.

README.md

@@ -13,6 +13,7 @@ It bundles a growing set of FreeBSD-derived userland tools into a single `vx` bi
 - `grep`, `egrep`, `fgrep`, `rgrep`
 - `find`
 - `xargs`
+- `lspci`, `setpci`
 
 ## Build
 
@@ -24,6 +25,14 @@ meson compile -C builddir
 meson test -C builddir
 ```
 
+To build only selected utilities:
+
+```text
+meson setup builddir -Dutils=grep,find,xargs
+```
+
+The default is `-Dutils=all`.
+
 ## Run
 
 You can invoke applets either through the multicall binary: