lalr1.cc: fix test suite portability

Reported by Rob Vermaas' Hydra build farm on x86_64-darwin 10.2.0 with
G++ 4.6.3.

* tests/headers.at (Several parsers): Include AT_DATA_SOURCE_PROLOGUE
in the files to compile.
* data/location.cc: Do not include twice string and iostream (once
by position.hh, and then by location.hh).
* README-hacking (Typical errors): Some hints for other maintainers.

tests/headers.at

@@ -249,6 +249,7 @@ for h in *.h *.hh
 do
   # No shell expansion with AT_DATA.
   cat >$h.cc <<EOF
+AT_DATA_SOURCE_PROLOGUE
 #include "$h"
 #include "$h"
 EOF