msvc: set the execution and source encoding to utf-8. Fixes #1294

gcc defaults to utf-8 for both (see -fexec-charset and -finput-charset in the
gcc man page) so we should use it with msvc as well.

msvc by default uses the locale encoding unless there is a BOM, see
https://msdn.microsoft.com/en-us/library/mt708821.aspx

meson.build

@@ -23,6 +23,8 @@ if cc.get_id() == 'msvc'
   # Disable SAFESEH with MSVC for plugins and libs that use external deps that
   # are built with MinGW
   noseh_link_args = ['/SAFESEH:NO']
+  # Set the input and exec encoding to utf-8, like is the default with GCC
+  add_project_arguments(cc.get_supported_arguments(['/utf-8']), language: 'c')
 else
   noseh_link_args = []
   # -mms-bitfields vs -fnative-struct ?