meson.build: Improve and fix Visual Studio builds

Hi,

Lately tepl gained a set of dependencies on ICU, so we need to make sure that we can find the ICU libraries on Visual Studio builds as well, since ICU's Visual Studio build files do not generate pkg-config files for us.

Also, this adds an option to disable introspection builds, as one might be cross-compiling tepl, where building the introspection files become impossible as the target binaries could not be run on the build system.

With blessings, thank you!