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.
11 jobs for meson.msvc in 12 minutes and 14 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5689916
gst-macos-10.15
meson macos

00:00:51

passed #5689918
meson-mingw-w64-build

00:04:20

manual #5689917
docker windows 1809 allowed to fail manual
meson msys2
passed #5689911
meson-shared-build

00:06:22

passed #5689912
meson-static-build

00:06:04

passed #5689914
docker windows 1809
meson vs2017 amd64

00:01:19

passed #5689915
docker windows 1809
meson vs2017 x86

00:01:25

passed #5689910
mingw-build

00:06:34

passed #5689908
shared-build

00:07:54

passed #5689909
static-build

00:06:21

failed #5689913
allowed to fail
meson-mingw-w64-build

00:06:12