tools: Make pd-tool build optional

pd-tool is only for posix-like systems, let it be disabled for other
platforms.

Change-Id: Idaa1b07882ff1c8143f6c71209240793f6e7f802
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
diff --git a/meson.options b/meson.options
index dea95f3..89f2309 100644
--- a/meson.options
+++ b/meson.options
@@ -23,3 +23,10 @@
     type: 'boolean',
     description: 'Enable transport implementation',
 )
+option(
+    'tools',
+    type: 'array',
+    description: 'Tools to build with the library',
+    choices: ['pd'],
+    value: ['pd'],
+)