meson: Don't use .def files for Visual Studio builds

Instead, when building with Visual Studio-style compilers, define 'FcPublic' as
appropriate so that symbols will be exported without the need to maintain a
.def file.

Fixes #245.
10 jobs for meson.msvc in 7 minutes and 37 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed meson macos #5595233
gst-macos-10.15

00:00:54

passed meson-mingw-w64-build #5595223

00:05:40

manual meson msys2 #5595236
docker windows 1809 allowed to fail manual
passed meson-shared-build #5595219

00:06:26

passed meson-static-build #5595221

00:06:21

passed meson vs2017 amd64 #5595224
docker windows 1809

00:01:24

passed meson vs2017 x86 #5595228
docker windows 1809

00:01:24

passed mingw-build #5595218

00:06:32

passed shared-build #5595214

00:07:37

passed static-build #5595216

00:06:19