Don't mandate dl functions for the extention build

Sqlite3-ruby doesn't use dl functions, nor does extconf compile sqlite3
sources, which possibly makes use of them.
So mandating dl functions in extconf is wrong and breaks the build on
Windows.

Fixes #250 .

This pertly reverts commit f4ffec2.

Author: larskanis

ext/sqlite3/extconf.rb

@@ -64,11 +64,7 @@ def asplode missing
 asplode('sqlite3.h')  unless find_header  'sqlite3.h'
 find_library 'pthread', 'pthread_create' # 1.8 support. *shrug*
 
-have_library 'dl'
-
-%w{ dlopen dlclose dlsym }.each do |func|
-  abort "missing function #{func}" unless have_func(func)
-end
+have_library 'dl' # for static builds
 
 if with_config('sqlcipher')
   asplode('sqlcipher') unless find_library 'sqlcipher', 'sqlite3_libversion_number'