build: list exported symbols explicitly

Instead of using a regex to match names to be exported from the libmpv
dynamic shared library, use a libmpv.def file, which lists all exported
functions explicitly.

This reduces the platform specifics in syms.py. I'm not sure if the
separate compile_sym task is still needed (it could probably be
collapsed, which would concentrate the platform specifics into one
place).

wscript_build.py

@@ -469,7 +469,7 @@ def build(ctx):
                 includes     = [ctx.bldnode.abspath(), ctx.srcnode.abspath()] + \
                                 ctx.dependencies_includes(),
                 features     = features,
-                export_symbols_regex = 'mpv_.*',
+                export_symbols_def = "etc/libmpv.def",
                 install_path = ctx.env.LIBDIR,
                 vnum         = libversion,
             )