… the support of Dbm in ocaml added some generic preprocessing in .mllib files so that this can be used for other optional dependencies too
…sal if dependencies available
Packaging scripts often use the DESTDIR=/some/directory variable to make the build system install the binaries in a specific directory which will be compressed to create the package. This commit adds support for this variable, by introducing a new INSTALL_DIR variable in the Makefile and replacing PREFIX by INSTALL_DIR everywhere it makes sense. Note that by default, INSTALL_DIR=$(PREFIX) so there should not be any compatibility problem with this commit.