CMake: Fix configuration on Win32

[myd7349]

On Windows with MSVC, this line:

check_library_exists(m pow "" WITH_HDRHISTOGRAM)

causes WITH_HDRHISTOGRAM set to OFF.
As a result:

• When specifying WITHOUT_WIN32_CONFIG=ON, the histogram source will not be built.
• When specifying WITHOUT_WIN32_CONFIG=OFF, there will be link erros since WITH_HISTOGRAM is defined as 1 in win32_config.h

References:
https://stackoverflow.com/questions/32816646/can-cmake-detect-if-i-need-to-link-to-libm-when-using-pow-in-c

[myd7349]

The configuration step on Win32 through CMake still have room for improvement. For example, the meaning of WITHOUT_WIN32_CONFIG is confusing
The doc string of this option is:

Avoid including win32_config.h on cmake builds

but rd.h will always include this header when built with MSVC.

I have a more aggressive patch here: myd7349@24965ee
If this patch is preferable, I'd like to add more commits to this PR.

[edenhill]

On MSVC we don't need to, and probably shouldnt, check for libm at all.
What do you think?

[myd7349]

@edenhill Thanks for your review! I will update this PR later.

[edenhill]

I think I like the more aggressive approach, better to get this all right in one go.

[myd7349]

Here is a related discussion about libm: mesonbuild/meson#5390 (comment)

[myd7349]

snappy support doesn't work out-of-box, reported by @rotrida:
microsoft/vcpkg#7456
To enable it, we have to pass an extra option:

cmake -DWITH_SNAPPY=ON

the problem is that, WITH_SNAPPY is not an option defined like this:

option(WITH_SNAPPY ...)

[edenhill]

@myd7349 WITH_SNAPPY should always be set to 1 in CMakeLists.txt