mkmf.rb: Refine message from `pkg_config`

lib/mkmf.rb

@@ -1849,6 +1849,12 @@ SRC
   # invoked with the options and a stripped output string is returned without
   # modifying any of the global values mentioned above.
   def pkg_config(pkg, *options)
+    fmt = "not found"
+    def fmt.%(x)
+      x ? x.inspect : self
+    end
+
+    checking_for "pkg-config for #{pkg}", fmt do
       _, ldir = dir_config(pkg)
       if ldir
         pkg_config_path = "#{ldir}/pkgconfig"
@@ -1911,6 +1917,7 @@ SRC
         nil
       end
     end
+  end
 
   # :stopdoc: