build: add cld2 option

Add an option for builders to explicitly disable cld2, instead of
relying on the automatic detection.

meson_options.txt

@@ -24,6 +24,11 @@ option('guile',
        value: 'auto',
        description: 'build the guile scripting support (requires guile-3.x)')
 
+option('cld2',
+       type : 'feature',
+       value: 'auto',
+       description: 'Compact Language Detector2')
+
 # by default, this uses guile_dep.get_variable(pkgconfig: 'extensiondir')
 option('guile-extension-dir',
        type: 'string',