meson: Add option for controlling PCH

In practice it turns out that turning off PCH is pretty
useful when debugging incorrect includes. I didn't want
to add this option initially because of the sheer amount
of copy-pasta that meson requires. But considering that
this is useful and the cost of copying the option a single
time is not too high this seems acceptable.

meson.options

@@ -27,3 +27,13 @@ option(
   value : false,
   description : 'Build benchmarks (requires gbenchmark)',
 )
+
+# Emulating 'auto' options, since we don't want auto_features to affect this
+# and there's no other way of conditionally defaulting the value.
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+)