lib/xiph/meson.build: define FLAC__NO_DLL for static libFLAC build (Windows)

In libFLAC 0.3.4 (commit c9530118a4), the "dllimport" check has been
changed from "_MSC_VER" to "_WIN32", and now the MPD build is affected
by it.

Defining FLAC__NO_DLL disables the use of "dllimport", which allows
linking properly to the static libFLAC build.

src/lib/xiph/meson.build

@@ -1,4 +1,11 @@
 libflac_dep = dependency('flac', version: '>= 1.2', required: get_option('flac'))
+
+if is_windows
+  # Our Windows build generates a static libFLAC build
+  libflac_dep = declare_dependency(compile_args: '-DFLAC__NO_DLL',
+                                   dependencies: libflac_dep)
+endif
+
 libopus_dep = dependency('opus', required: get_option('opus'))
 
 if get_option('tremor').enabled()