meson: Fix Python library searching on Windows

Neither LIBDIR nor LIBPL are set with the native windows Python
(unlike MSYS2), so we need to use `prefix` which takes us to the
rootdir of the Python installation.

The name is also different: it's python312.dll, not python3.12.dll.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6734>

subprojects/gst-python/meson.build

@@ -45,39 +45,53 @@ pylib_loc = get_option('libpython-dir')
 fsmod = import('fs')
 pylib_prefix = 'lib'
 pylib_suffix = 'so'
+pylib_ver = python_dep.version()
+pylib_locs = []
 if host_system == 'windows'
   if cc.get_argument_syntax() == 'msvc'
     pylib_prefix = ''
   endif
   pylib_suffix = 'dll'
+  pylib_ver = pylib_ver.replace('.', '')
 elif host_system == 'darwin'
   pylib_suffix = 'dylib'
 endif
 pylib_fnames = []
 # Library name with soversion, non-devel package
 if python.has_variable('INSTSONAME')
+  # For example, libpython3.12.so.1.0 (Linux), libpython3.11.dll.a (MSYS2), etc.
   pylib_fnames += python.get_variable('INSTSONAME')
 endif
 # Library name without soversion, devel package, framework, etc.
 if python.has_variable('LDLIBRARY')
+  # For example, libpython3.12.so (Linux), libpython3.11.dll.a (MSYS2), etc.
   pylib_fnames += python.get_variable('LDLIBRARY')
 endif
 # Manually construct name as a fallback
 pylib_fnames += [
-  pylib_prefix + 'python' + python_dep.version() + python_abi_flags + '.' + pylib_suffix
+  pylib_prefix + 'python' + pylib_ver + python_abi_flags + '.' + pylib_suffix
 ]
 if pylib_loc != ''
   pylib_locs = [pylib_loc]
 else
-  pylib_locs = [
+  if python.has_variable('LIBDIR')
-    python.get_variable('LIBDIR', ''),
+    pylib_locs += python.get_variable('LIBDIR')
-    python.get_variable('LIBPL', ''),
+  endif
-  ]
+  if python.has_variable('LIBPL')
+    pylib_locs += python.get_variable('LIBPL')
+  endif
+  # On Windows, python312.dll is in the rootdir where Python is installed,
+  # which is configured as the "prefix" in sysconfig.
+  if host_system == 'windows'
+    pylib_locs += python.get_variable('prefix')
+  endif
 endif
 pylib_fname = ''
 foreach loc: pylib_locs
   foreach fname: pylib_fnames
-    if fsmod.exists(loc / fname)
+    fpath = loc / fname
+    debug(f'Looking for Python library at: @fpath@')
+    if fsmod.exists(fpath)
       pylib_fname = fname
       message(f'PY_LIB_FNAME = @fname@ (@loc@)')
       break