Configurable location for extension .control files

Hello,

I am working with the NixOS Linux Distribution [nixos], which has a 
fairly radical approach to package management. If you aren't familiar 
with it, essentially all packages are installed in isolation - such that 
packages cannot interfere with each other.

This is causing a bit of a problem with PostgreSQL extensions that are 
usually installed via CREATE EXTENSION. For extensions to be used, a 
.control file must be placed in SHAREDIR/extension, but this is not 
possible under NixOS as once PostgreSQL is installed that directory is 
essentially immutable.

What wolud work best for us is to allow this path to be configurable, 
ideally through either an environment variable, command line switch, or 
(and this is the least desirable) a postgresql.conf option.

Would love to hear your thoughts. Once I get confirmation on the best 
approach to take, I can try my hand at providing a patch.

Thanks,
- Ollie (ocharles)

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[nixos]: http://nixos.org