Only set CMAKE_GENERATOR_PLATFORM to "x64" for 64 bit toolchains for any
of the "Visual Studio" generators, which use msbuild.
For other generators such as Ninja or NMake (jom) set
CMAKE_C[XX]_COMPILER to "cl" to let cmake find the compiler in the
user's PATH and set up the toolchain appropriately. On the user's part,
all that is required is running the 64 bit dev tools command prompt, or
setting up the equivalent environment in powershell.
We are no longer restricted to msbuild, and other generators such as
Ninja will work correctly.
Switch the appveyor job to Ninja, this requires some wrangling because
the appveyor environment does not include ninja. I used this:
https://github.com/boostorg/hana/blob/master/.appveyor.yml
Also update the README.md instructions to use Ninja.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>
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