test_extlibs.rb: show mkmf.log

* test/ruby/test_extlibs.rb (check_existence): show mkmf.log of
  failed extension libraries.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49045 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_extlibs.rb

@@ -1,11 +1,19 @@
 require "envutil"
 
 class TestExtLibs < Test::Unit::TestCase
+  @extdir = $".grep(/\/rbconfig\.rb\z/) {break "#$`/ext"}
+
   def self.check_existence(ext, add_msg = nil)
     add_msg = ".  #{add_msg}" if add_msg
+    log = "#{@extdir}/#{ext}/mkmf.log"
     define_method("test_existence_of_#{ext}") do
       assert_separately([], <<-"end;", ignore_stderr: true) # do
-        assert_nothing_raised("extension library `#{ext}' is not found#{add_msg}") do
+        log = #{log.dump}
+        msg = proc {
+          "extension library `#{ext}' is not found#{add_msg}\n" <<
+            (File.exist?(log) ? File.read(log) : "\#{log} not found")
+        }
+        assert_nothing_raised(msg) do
           require "#{ext}"
         end
       end;