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.

meson_options.txt

@@ -0,0 +1,20 @@
+option('utils',
+  type : 'array',
+  value : ['all'],
+  choices : [
+    'all',
+    'patch',
+    'diff',
+    'cmp',
+    'diff3',
+    'sdiff',
+    'which',
+    'gzip',
+    'grep',
+    'find',
+    'xargs',
+    'lspci',
+    'setpci',
+  ],
+  description : 'Utilities to include in vx (default: all)'
+)