get rid of names, otherwise the returned vector will have names like this:

         pdfcrop.pdfcrop            ghostcript.gs
"/usr/local/bin/pdfcrop"      "/usr/local/bin/gs"

this is an interesting "feature" of c()

Author: yihui

R/util.R

@@ -314,8 +314,8 @@ find_program <- function(program) {
 
 has_crop_tools <- function() {
   tools <- c(
-    pdfcrop = find_program("pdfcrop"),
-    ghostcript = tools::find_gs_cmd()
+    pdfcrop = unname(find_program("pdfcrop")),
+    ghostcript = unname(tools::find_gs_cmd())
   )
   missing <- tools[tools == ""]
   if (length(missing) == 0) return(TRUE)