Sort objects to make build reproducible

--- a/Makefile
+++ b/Makefile
@@ -35,18 +35,18 @@ EXT_LONGVER = $(shell grep '"version":'
 EXT_SHORTVER = $(shell grep 'default_version' $(EXTENSION).control | head -1 | sed -e "s/default_version\s*=\s'\(.*\)'/\1/")
 PG91 = $(shell $(PG_CONFIG) --version | grep -qE " 8\.| 9\.0" && echo pre91 || echo 91)
 
-SRC_C = $(wildcard src/*.c)
-SRC_H = $(wildcard src/*.h)
+SRC_C = $(sort $(wildcard src/*.c))
+SRC_H = $(sort $(wildcard src/*.h))
 SRCFILES = $(SRC_C) $(SRC_H)
 OBJS = $(patsubst %.c,%.o,$(SRC_C))
 TESTFILES = $(wildcard test/sql/*.sql) $(wildcard test/expected/*.out)
-DOCS = $(wildcard docs/*.rst)
+DOCS = $(sort $(wildcard docs/*.rst))
 
 PKGFILES = AUTHORS COPYING README.rst Makefile \
 	META.json pgmp.control \
 	sql/pgmp.pysql sql/uninstall_pgmp.sql \
 	$(SRCFILES) $(DOCS) $(TESTFILES) \
-	$(wildcard tools/*.py)
+	$(sort $(wildcard tools/*.py))
 
 ifeq ($(PG91),91)
 INSTALLSCRIPT=sql/pgmp--$(EXT_SHORTVER).sql
