Fix some 32-bit builds

Perl on some 32-bit systems needs -D_FILE_OFFSET_BITS=64.  See also commit
02f1363e.

perl/Makefile.am

@@ -22,7 +22,8 @@ libNixStore_la_LIBADD = $(top_builddir)/src/libstore/libstore.la
 
 AM_CXXFLAGS = \
   -I$(top_srcdir)/src -I$(top_srcdir)/src/libutil -I$(top_srcdir)/src/libstore \
-  -I$(shell $(perl) -e 'use Config; print $$Config{archlibexp};')/CORE
+  -I$(shell $(perl) -e 'use Config; print $$Config{archlibexp};')/CORE \
+  -D_FILE_OFFSET_BITS=64
 
 lib/Nix/Store.cc: lib/Nix/Store.xs
 	xsubpp $^ -output $@