Fix the install rule for snowball_create.sql.

This file could be in the current (build) directory if we just
built it.  However, when installing from a VPATH build from a
tarball, it will exist in the source directory and gmake will
therefore not rebuild it.  Use the $< macro to find out where
gmake found it.

Oversight in b3a0d8324, which also exposes a buildfarm testing gap:
we test install from VPATH builds from bare source trees, but not
from tarballs.

Per report from Christoph Berg.

Discussion: https://postgr.es/m/ZGzEAqjxkkoY3ooH@msg.df7cb.de

src/backend/snowball/Makefile

@@ -106,10 +106,13 @@ $(SQLSCRIPT): snowball_create.pl snowball_func.sql.in snowball.sql.in
 
 distprep: $(SQLSCRIPT)
 
-install: all installdirs install-lib
-	$(INSTALL_DATA) $(SQLSCRIPT) '$(DESTDIR)$(datadir)'
+install: all installdirs install-lib install-script
 	$(INSTALL_DATA) $(addprefix $(srcdir)/stopwords/,$(stop_files)) '$(DESTDIR)$(datadir)/$(DICTDIR)'
 
+# $(SQLSCRIPT) might be in the srcdir or the build dir
+install-script: $(SQLSCRIPT)
+	$(INSTALL_DATA) $< '$(DESTDIR)$(datadir)'
+
 installdirs: installdirs-lib
 	$(MKDIR_P) '$(DESTDIR)$(datadir)' '$(DESTDIR)$(datadir)/$(DICTDIR)'