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 8 minutes and 12 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed meson macos #4801130
gst-macos-10.15

00:00:49

passed meson-mingw-w64-build #4801127

00:04:01

manual meson msys2 #4801131
docker windows 1809 allowed to fail manual
passed meson-shared-build #4801125

00:02:54

passed meson-static-build #4801126

00:02:51

passed meson vs2017 amd64 #4801128
docker windows 1809

00:01:05

passed meson vs2017 x86 #4801129
docker windows 1809

00:01:08

passed mingw-build #4801124

00:06:20

passed shared-build #4801122

00:08:12

passed static-build #4801123

00:05:31