Changes

Introduce Makefile.build_config.in

This moves the configure-generated parts of Makefile.common to a
separate (generated) Makefile, allowing Makefile.common to be a normal
Makefile.

OCaml's build system Makefile's now include Makefile.build_config (which
itself includes Makefile.config) but Makefile.config is still installed
as before. This allows configure to generate variables which are
specific to the build process and are not intended to be exported to the
installation.
 (commit: 3a40b2f)

Use autoconf AC_PROG_INSTALL results

Instead of hand-written INSTALL_{DATA,PROG}. autoconf "does the right
thing" to provide a command which works as expected.

See
  https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Particular-Programs.html#index-AC_005fPROG_005fINSTALL-269
 (commit: be6eb2c)

Cease committing C dependendency information

When building for the first time, the only requirement is that generated
header files have been built (jumptbl.h, version.h and opnames.h).
Detailed dependency information is only required when headers have been
edited.

COMPUTE_DEPS in Makefile.config controls whether C dependency
information should be generated on a per-file basis. This variable is
controlled by a new --disable-dependency-generation in configure which
is enabled for Git checkouts and disabled for tarballs (i.e. releases).

The Microsoft C compiler (cl) cannot generate dependencies in a
consistent way which we can consume, so for a Git checkout configure
searches for an additional C compiler in order to compute dependencies.
This is obviously not required for a user-build.

As a result, the MSVC port can now safely run make alldepend, since only
OCaml dependency information is committed to the repo after this change.

CI does not need to waste time testing the dependency information,
because it only tests a single build. A single Travis job has been added
which tests the build system code to generate the dependency information
(and provides a single `make -j` run in CI, although Inria's CI also
tests parallel building continuously).
 (commit: ac2a9dd)

Reformat .travis.yml env blocks
 (commit: fb375c5)