Use cli::is_dynamic_tty() do decide on dynamic output

This can be forced or suppressed with the `cli.dynamic`
option and the `R_CLI_DYNAMIC` env var. See
`?cli::is_dynamic_tty` for details.

Now pkgbuild output looks OK in Rmds, by default.

Closes #64.

R/callback.R

@@ -105,3 +105,10 @@ block_callback <- function(quiet) {
 is_new_check <- function(x) {
   grepl("^\\* ", x)
 }
+
+simple_callback <- function(quiet) {
+  function(x) {
+    if (quiet) return()
+    cat(x)
+  }
+}

R/rcmd.R

@@ -23,9 +23,15 @@ rcmd_build_tools <- function(..., env = character(), required = TRUE, quiet = FA
 
   warn_for_potential_errors()
 
+  callback <- if (cli::is_dynamic_tty()) {
+    block_callback(quiet)
+  } else {
+    simple_callback(quiet)
+  }
+
   res <- with_build_tools(
     callr::rcmd_safe(..., env = env, spinner = FALSE, show = FALSE,
-      echo = FALSE, block_callback = block_callback(quiet), stderr = "2>&1"),
+      echo = FALSE, block_callback = callback, stderr = "2>&1"),
     required = required
   )