Err on undeclared functions

Stop at compilation time instead of run time. This gives especially
helpful, instant feedback when testing the code for 1.8 as a lot of
the C API has changed between 1.8 and 2.0.

ext/yajl/extconf.rb

@@ -2,6 +2,6 @@
 require 'rbconfig'
 
 $CFLAGS << ' -Wall -funroll-loops'
-$CFLAGS << ' -Wextra -O0 -ggdb3' if ENV['DEBUG']
+$CFLAGS << ' -Werror-implicit-function-declaration -Wextra -O0 -ggdb3' if ENV['DEBUG']
 
 create_makefile('yajl/yajl')