[do_postgress] Get ext to build on Ruby 2.0.0 (stock OS X).

[alloy]

For some reason the c.h PG header is not being included when building on older Rubies. Explicitely adding the base postgres.h header which includes the c.h header fixes it.

Here are the build logs before and after https://gist.github.com/alloy/23a6b235d35a346f4415

[alloy]

FYI This is the header in question https://github.com/postgres/postgres/blob/master/src/include/c.h

[floere]

This is a lovely PR!

[neonichu]

:+1: