meson.build: drop config.h.meson

This seems to be a leftover from the autotools conversion but it
does not provide huge benefits. Meson's default behavior is to generate a
header like this, the main advantage of a config.h.meson header is to *filter*
things that should show up in the config.h file. That comes at the cost of
having to #mesondefine every single variable we want.

In most cases, it's easier with meson to just use different configuration_data
objects instead - which we do for most of the code.

The new header file is identical to the old one, discounting comments, sort
order and the following #defines that didn't end up in the config.h before:
  HAVE_STRNDUPA
  HAVE_STDDEF_H

meson.build

@@ -394,9 +394,8 @@ if not get_option('tests').disabled()
   subdir('test')
 endif
 
-configure_file(input : 'config.h.meson',
-  output : 'config.h',
-  configuration : cdata)
+configure_file(output : 'config.h',
+               configuration : cdata)
 
 if not get_option('pipewire-jack').disabled()
   subdir('pipewire-jack')