Make system is broke.

[EvanCarroll]

This doesn't actually work for me,

ifeq ($(PG91),yes)                                                  
#STUFF CUT OUT...
DATA = $(wildcard sql/*--*.sql) sql/$(EXTENSION)--$(EXTVERSION).sql 
## STUFF
endif                                                               

By that I mean it generates the error

/usr/bin/install: will not overwrite just-created '/usr/share/postgresql/9.5/extension/bignum--0.8.0.sql' with './/sql/bignum--0.8.0.sql'
/usr/lib/postgresql/9.5/lib/pgxs/src/makefiles/pgxs.mk:114: recipe for 
target 'install' failed
make: *** [install] Error 1

Adding some debug $(info $$DATA is [$(DATA)]) in ifeq ($(PG91),yes) I see

$DATA is [sql/bignum--0.8.0.sql sql/bignum--0.8.0.sql]

And, that's what we're setting it too

DATA = $(wildcard sql/*--*.sql) sql/$(EXTENSION)--$(EXTVERSION).sql

That causes bignum--0.8.0.sql to be copied twice to the same location which triggers this error. Can we please just nuke the 9.1 stuff: 9.1 and 8.x are no longer supported, this isn't needed nor is it worth the added complexity.

You can see this pattern here. I think you copied it wrong. In their pg91 section they only have

DATA = $(wildcard sql/*--*.sql)