Provide for contrib and pgxs modules to install include files.

This allows out-of-tree PLs and similar code to get access to
definitions needed to work with extension data types.

The following existing modules now install headers: contrib/cube,
contrib/hstore, contrib/isn, contrib/ltree, contrib/seg.

Discussion: https://postgr.es/m/87y3euomjh.fsf%40news-spur.riddles.org.uk

contrib/cube/Makefile

@@ -9,6 +9,8 @@ DATA = cube--1.2.sql cube--1.2--1.3.sql cube--1.3--1.4.sql \
 	cube--unpackaged--1.0.sql
 PGFILEDESC = "cube - multidimensional cube data type"
 
+HEADERS = cubedata.h
+
 REGRESS = cube
 
 EXTRA_CLEAN = y.tab.c y.tab.h

contrib/hstore/Makefile

@@ -11,6 +11,8 @@ DATA = hstore--1.4.sql hstore--1.4--1.5.sql \
 	hstore--unpackaged--1.0.sql
 PGFILEDESC = "hstore - key/value pair data type"
 
+HEADERS = hstore.h
+
 REGRESS = hstore
 
 ifdef USE_PGXS

contrib/isn/Makefile

@@ -7,6 +7,9 @@ DATA = isn--1.1.sql isn--1.1--1.2.sql \
 	isn--1.0--1.1.sql isn--unpackaged--1.0.sql
 PGFILEDESC = "isn - data types for international product numbering standards"
 
+# the other .h files are data tables, we don't install those
+HEADERS_isn = isn.h
+
 REGRESS = isn
 
 ifdef USE_PGXS

contrib/ltree/Makefile

@@ -9,6 +9,8 @@ EXTENSION = ltree
 DATA = ltree--1.1.sql ltree--1.0--1.1.sql ltree--unpackaged--1.0.sql
 PGFILEDESC = "ltree - hierarchical label data type"
 
+HEADERS = ltree.h
+
 REGRESS = ltree
 
 ifdef USE_PGXS

contrib/seg/Makefile

@@ -8,6 +8,8 @@ DATA = seg--1.1.sql seg--1.1--1.2.sql seg--1.2--1.3.sql \
 	seg--1.0--1.1.sql seg--unpackaged--1.0.sql
 PGFILEDESC = "seg - line segment data type"
 
+HEADERS = segdata.h
+
 REGRESS = seg
 
 EXTRA_CLEAN = y.tab.c y.tab.h