Add conf-msvc32.1 and conf-msvc64.1
These packages can only be installed if a suitable Visual Studio
installation containing the Microsoft C compiler and macro assembler can
be found.
The two packages use msvs-detect to construct a .config file containing
the required environment variable changes necessary to make this
compiler available. These variables can then be used in the build-env or
setenv fields of dependent packages.
Both packages can be installed simultaneously; dependent packages are
required to ensure that only one toolchain is actually in the
environment at any given time.
The packages fail to install if a compiler cannot be found - opam
presently lacks an expressive enough depext system to be able to do
anything further.