Replace ocamlbuild by a second-stage makefile to compile the OCaml code and produce the executables.
configure: add check for GNU make.
Use String.map instead of reimplementing it ourselves.
Avoids warnings with 4.02.
Remove ocamlbuild configuration files, no longer used.
Use OCaml's .opt compilers when available.
Cleanups in configure.
Merge branch 'master' into pure-makefiles
Minor bug fixes in configure and Makefile.extr
Merge pull request #3 from AbsInt/pure-makefiles

Merge of the pure-makefiles branch, which uses Makefiles instead of ocamlbuild to build the Caml code.