Fix out of source build

When included into postgresql contribs, and postgresql configured "out of source", pg_pathman could not build because it misses include path.

Fix it by referring top_srcdir and subdir if build without USE_PGXS.

Author: funny-falcon

Makefile

@@ -10,7 +10,11 @@ OBJS = src/init.o src/relation_info.o src/utils.o src/partition_filter.o \
 	src/compat/pg_compat.o src/compat/relation_tags.o src/compat/rowmarks_fix.o \
 	$(WIN32RES)
 
+ifdef USE_PGXS
 override PG_CPPFLAGS += -I$(CURDIR)/src/include
+else
+override PG_CPPFLAGS += -I$(top_srcdir)/$(subdir)/src/include
+endif
 
 EXTENSION = pg_pathman