gettext: prefer POTFILES.in if it exists

If POTFILES.in exists, then it will have come from autotools, in which
case it is explicitly the file passed to xgettext -f, and the POTFILES
file itself is generated by autotools as a proxy file which eventually
gets inlined into the final Makefile as a variable "POTFILES = ......"

In this case, attempting to use POTFILES as the input file will simply
result in syntax errors and the inability to find files with a literal
trailing " \" in the name. Usually POTFILES will not exist at all, and
we would fallback on POTFILES.in, but if the source tree happens to be
dirty, this would result in errors. Since it's never going to be right
to use it, we can just do the right thing from the start and carry on.

mesonbuild/scripts/gettext.py

@@ -44,9 +44,9 @@ def read_linguas(src_sub):
         return []
 
 def run_potgen(src_sub, pkgname, datadirs, args):
-    listfile = os.path.join(src_sub, 'POTFILES')
+    listfile = os.path.join(src_sub, 'POTFILES.in')
     if not os.path.exists(listfile):
-        listfile = os.path.join(src_sub, 'POTFILES.in')
+        listfile = os.path.join(src_sub, 'POTFILES')
         if not os.path.exists(listfile):
             print('Could not find file POTFILES in %s' % src_sub)
             return 1