Teach plpgsql's "make clean" to remove generated test files.

Copy the rules that src/test/regress/GNUmakefile uses for this purpose.
Since these files are .gitignore'd, the mistake wasn't obvious unless
you happened to look at "git status --ignored" in an allegedly clean
tree.

Oversight in commit 1858b105b.  No need for back-patch since that's
not in the back branches.

src/pl/plpgsql/src/Makefile

@@ -41,6 +41,11 @@ TOOLSDIR = $(top_srcdir)/src/tools
 GEN_KEYWORDLIST = $(PERL) -I $(TOOLSDIR) $(TOOLSDIR)/gen_keywordlist.pl
 GEN_KEYWORDLIST_DEPS = $(TOOLSDIR)/gen_keywordlist.pl $(TOOLSDIR)/PerfectHash.pm
 
+# Test input and expected files.  These are created by pg_regress itself, so we
+# don't have a rule to create them.  We do need rules to clean them however.
+input_files = $(patsubst $(srcdir)/input/%.source,sql/%.sql, $(wildcard $(srcdir)/input/*.source))
+output_files := $(patsubst $(srcdir)/output/%.source,expected/%.out, $(wildcard $(srcdir)/output/*.source))
+
 all: all-lib
 
 # Shared library stuff
@@ -111,6 +116,7 @@ distprep: pl_gram.h pl_gram.c plerrcodes.h pl_reserved_kwlist_d.h pl_unreserved_
 # are not cleaned here.
 clean distclean: clean-lib
 	rm -f $(OBJS)
+	rm -f $(output_files) $(input_files)
 	rm -rf $(pg_regress_clean_files)
 
 maintainer-clean: distclean