Install rum.h

There could be other extension defining opclasses for RUM.  Such extensions
would need rum.h.  So, we need to install it.  Unfortunately, pgxs.mk don't have
variable for this.  Thus, we define it as new target in Makefile.

Author: Alexander Korotkov

Diff for: Makefile

@@ -10,6 +10,7 @@ OBJS = src/rumsort.o src/rum_ts_utils.o src/rumtsquery.o \
 EXTENSION = rum
 DATA = rum--1.0.sql
 PGFILEDESC = "RUM index access method"
+INCLUDES = src/rum.h
 
 REGRESS = rum rum_hash ruminv timestamp orderby orderby_hash altorder \
 	altorder_hash limits
@@ -29,3 +30,8 @@ endif
 
 wal-check: temp-install
 	$(prove_check)
+
+install: installincludes
+
+installincludes:
+	$(INSTALL_DATA) $(addprefix $(srcdir)/, $(INCLUDES)) '$(DESTDIR)$(includedir)/'