Trying to use PostGIS with Postgres: "ERROR: could not open extension control file"

Question

I've just installed Postgres 9.3.4 and PostGIS 2.1 on OSX Mavericks using the very latest versions of the KyngChaos libraries.

However, when I try to create spatial extensions on a database, I can't, because Postgres claims not to be able to see the extension file:

:~ anna$ psql -d land -c "CREATE EXTENSION postgis;"
ERROR:  could not open extension control file 
"/usr/local/Cellar/postgresql/9.3.4/share/postgresql/extension/postgis.control": 
No such file or directory

It looks as though the file is located somewhere else:

:~ anna$ mdfind postgis.control
/usr/local/pgsql-9.3/share/extension/postgis.control

I'm definitely using the expected version of Postgres, although I don't know if this is where the KyngChaos libraries usually install to:

Annas-MacBook-Air:~ anna$ psql --version
psql (PostgreSQL) 9.3.4
Annas-MacBook-Air:~ anna$ which psql
/usr/local/pgsql-9.3/bin/psql

I've been quite careful not to install Postgres using Homebrew etc because I know that it can cause problems having multiple versions of the same system.

Can anyone advise on why the extension file isn't in the expected place?

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UPDATE:

It looks like perhaps there are multiple versions of Postgres floating around on my system, and I'm using the wrong one?

:~ anna$ mdfind -name "postgres" | grep -G "postgres$"
/usr/local/pgsql-9.3/bin/postgres
/usr/local/var/postgres
/usr/local/Cellar/postgresql/9.3.4/bin/postgres

Which one should I be using, and how can I set the system to use this one by default?

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FURTHER UPDATE

Here's the output of pg_config:

BINDIR = /usr/local/pgsql-9.3/bin
DOCDIR = /usr/local/pgsql-9.3/share/doc
INCLUDEDIR = /usr/local/pgsql-9.3/include
PKGINCLUDEDIR = /usr/local/pgsql-9.3/include
INCLUDEDIR-SERVER = /usr/local/pgsql-9.3/include/server
LIBDIR = /usr/local/pgsql-9.3/lib
PKGLIBDIR = /usr/local/pgsql-9.3/lib
LOCALEDIR =
MANDIR = /usr/local/pgsql-9.3/man
SHAREDIR = /usr/local/pgsql-9.3/share
SYSCONFDIR = /usr/local/pgsql-9.3/etc
PGXS = /usr/local/pgsql-9.3/lib/pgxs/src/makefiles/pgxs.mk
CONFIGURE = '--with-openssl' '--with-pam' '--with-krb5' '--with-gssapi' '--with-ldap' '--enable-thread-safety' '--with-bonjour' '--with-python' '--without-perl' '--enable-nls' '--with-libxml' 'CC=clang' 'CXX=clang++' 'CFLAGS=-Os -arch x86_64 -isysroot /Developer/SDKs/MacOSX10.7.sdk -D_FILE_OFFSET_BITS=64' 'LD=clang' 'LDLFAGS=-arch x86_64 -isysroot /Developer/SDKs/MacOSX10.7.sdk'
CC = gcc
CPPFLAGS = -I/usr/include/libxml2
CFLAGS = -Os -arch x86_64 -D_FILE_OFFSET_BITS=64 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -fno-strict-aliasing -fwrapv
CFLAGS_SL =
LDFLAGS = -arch x86_64 -Wl,-dead_strip_dylibs
LDFLAGS_SL =
LIBS = -lintl -liconv -lxml2 -lpam -lssl -lcrypto -lgssapi_krb5 -ledit -lz
VERSION = PostgreSQL 9.3.4

Solution

Searching a bit around I found this GIST where user jordanmkoncz gives a solution for OSX:

1. Use homebrew to install PostGIS.
2. You need to be careful because brew will install the latest available version of PostGIS which may not be compatible with your specific version of PostgreSQL. Here is a compatibility table for your convenience.

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To install a specific version of PostGIS (or any other formula) jordnmkoncz recommends using this method: Homebrew install specific version of formula? for installing spec

I had the same problem with Linux Mint (Ubuntu like) which I solve following this gis.stackexchange thread.