Improve Meson builds on MSVC

Hi,

As pointed out by @kjellahl in !5 (merged), the assert messages where the .lib files for glibmm, cairomm and sigc++-3.x are not found are not correctly formatted, so this MR is opened to fix that, as well as make it clearer to people that the appropriate .lib files for these 3 libraries are all required.

Also add instructions in README.win32 to let people know how MSVC builds can be done using Meson.

Note that in this MR and in !5 (merged), the generated pkg-config files for pangomm only includes the pangocairo dependency, but not glibmm and cairomm (and implicitly, libsigc++-3.x). The rationale is to have these libraries eventually buildable by Meson and so let the dependency discovery and pkg-config file depend on these as they come in, since MSVC (NMake) builds of the -mm libraries do not use nor generate pkg-config files.

With blessings, thank you!

With blessings, thank you!