Ignore compiler warnings that are expected

This adds the following compiler warning ignores, to reduce noise:
-Wno-shorten-64-to-32
-Wno-missing-noreturn
-Wno-incompatible-pointer-types-discards-qualifiers

All of these occur in extracted/generated code, and as such cannot easily
be resolved.

Author: lfittl

ext/pg_query/extconf.rb

@@ -8,7 +8,7 @@
 $objs = Dir.glob(File.join(__dir__, '*.c')).map { |f| Pathname.new(f).sub_ext('.o').to_s }
 
 if RUBY_PLATFORM !~ /cygwin|mswin|mingw|bccwin|wince|emx/
-  $CFLAGS << " -fvisibility=hidden -O3 -Wall -fno-strict-aliasing -fwrapv -fstack-protector -Wno-unused-function -Wno-unused-variable -Wno-clobbered -Wno-sign-compare -Wno-discarded-qualifiers -Wno-unknown-warning-option -g"
+  $CFLAGS << " -fvisibility=hidden -O3 -Wall -fno-strict-aliasing -fwrapv -fstack-protector -Wno-unused-function -Wno-unused-variable -Wno-clobbered -Wno-sign-compare -Wno-discarded-qualifiers -Wno-unknown-warning-option -Wno-shorten-64-to-32 -Wno-missing-noreturn -Wno-incompatible-pointer-types-discards-qualifiers -g"
 end
 
 $INCFLAGS = "-I#{File.join(__dir__, 'include')} " + "-I#{File.join(__dir__, 'include', 'postgres')} " + $INCFLAGS