libgimp: don't run defcheck.py on Windows.

Ironically, it is a test for the Windows platform but it cannot run on
Windows. First, because it expects a .so (which could be easily fixed),
but even more because from web search, it looks like the nm tool may not
exist on Windows (though I haven't checked).

Anyway we only ever ran it from Linux machines and up to now, it worked
just fine and was useful anyway. So let's go with it.

Also clean a bit remnants from older attempts to run this script.

libgimp/meson.build

@@ -519,8 +519,14 @@ else
   endif
 endif
 
-if have_python
-  # Check .def files for Windows.
+if have_python and not platform_windows
+  # Verify .def files for Windows.
+  # Ironically we only check this on non-Windows platform, since the
+  # script expects .so libraries, and I'm not sure that the `nm` tool is
+  # available on Windows.
+  # On the other hand, it is how it always has been, since we used to
+  # only run this on dist step (which we historically run on a GNU/Linux
+  # machine). And it worked fine until now.
   custom_target('check-def-files',
     input: [
       '../libgimpbase/gimpbase.def',

meson.build

@@ -1178,7 +1178,6 @@ localedir      = get_option('datadir') / 'locale'
 
 
 # Check for internal tools
-defcheck            = find_program('tools'/'defcheck.py')
 extract_vector_icon = find_program('tools'/'extract-vector-icon.sh')
 generate_changelog  = find_program('tools'/'generate_changelog.sh')
 generate_news       = find_program('tools'/'generate-news')
@@ -1625,19 +1624,6 @@ if xmllint.found()
   )
 endif
 
-if have_python
-  # TODO this python script is waiting for autotools directory/files structure
-  custom_target('check-defs',
-    input : [ ],
-    output: [ 'check-defs', ],
-    command: [
-      find_program(meson.source_root() / 'tools' / 'defcheck.py'),
-      meson.source_root(),
-    ],
-    build_by_default: false,
-  )
-endif
-
 custom_target('Changelog',
   input : [ ],
   output: [ 'Changelog', ],