zlib: no checks [ci skip]

* ext/zlib/extconf.rb: no feature checks when building zlib from
  the source, assume it is recent.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56220 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/zlib/extconf.rb

@@ -70,9 +70,15 @@ if have_zlib
 
   $defs.concat(defines.collect{|d|' -D'+d})
 
-  have_func('crc32_combine', 'zlib.h')
-  have_func('adler32_combine', 'zlib.h')
-  have_type('z_crc_t', 'zlib.h')
+  if zsrc
+    $defs << "-DHAVE_CRC32_COMBINE"
+    $defs << "-DHAVE_ADLER32_COMBINE"
+    $defs << "-DHAVE_TYPE_Z_CRC_T"
+  else
+    have_func('crc32_combine', 'zlib.h')
+    have_func('adler32_combine', 'zlib.h')
+    have_type('z_crc_t', 'zlib.h')
+  end
 
   create_makefile('zlib') {|conf|
     if zsrc