A few suffix fixes.

dependencies.py

@@ -137,7 +137,7 @@ class ExternalProgram():
             self.fullpath = [shutil.which(name)]
             if self.fullpath[0] is None and search_dir is not None:
                 trial = os.path.join(search_dir, name)
-                suffix = os.path.splitext(trial)[-1].lower()
+                suffix = os.path.splitext(trial)[-1].lower()[1:]
                 if environment.is_windows() and (suffix == 'exe' or suffix == 'com'\
                                           or suffix == 'bat'):
                     self.fullpath = [trial]

meson_install.py

@@ -107,7 +107,7 @@ def run_install_script(d):
     script = d.install_script
     print('Running custom install script %s' % script)
     suffix = os.path.splitext(script)[1].lower()
-    if platform.system().lower() == 'windows' and suffix != 'bat':
+    if platform.system().lower() == 'windows' and suffix != '.bat':
         first_line = open(script).readline().strip()
         if first_line.startswith('#!'):
             commands = first_line[2:].split('#')[0].strip().split()