Fix 1.8 support

We need to check for libpthread when compiling.  Ruby >=1.9 already
links against pthread, so it would Just Work (tm) but 1.8 would not.
Considering how long this is broken with no one complaining, why do we still
support 1.8?

Author: tenderlove

ext/sqlite3/extconf.rb

@@ -28,6 +28,7 @@ def asplode missing
 end
 
 asplode('sqlite3.h')  unless find_header  'sqlite3.h'
+find_library 'pthread', 'pthread_create' # 1.8 support. *shrug*
 asplode('sqlite3') unless find_library 'sqlite3', 'sqlite3_libversion_number'
 
 # Functions defined in 1.9 but not 1.8