qt: Fix detection of tools on Windows

When you pass an absolute path to shutil.which, it will not implicitly
append any extensions. This is problematic on Windows, so we need to
account for that.

This fixes detection of Qt tools on Windows which are searched with the
full path to the Qt bindir.

mesonbuild/dependencies.py

@@ -373,6 +373,8 @@ class WxDependency(Dependency):
         return self.is_found
 
 class ExternalProgram():
+    windows_exts = ('exe', 'com', 'bat')
+
     def __init__(self, name, fullpath=None, silent=False, search_dir=None):
         self.name = name
         if fullpath is not None:
@@ -410,11 +412,10 @@ class ExternalProgram():
             pass
         return False
 
-    @staticmethod
-    def _is_executable(path):
+    def _is_executable(self, path):
         suffix = os.path.splitext(path)[-1].lower()[1:]
         if mesonlib.is_windows():
-            if suffix == 'exe' or suffix == 'com' or suffix == 'bat':
+            if suffix in self.windows_exts:
                 return True
         elif os.access(path, os.X_OK):
             return True
@@ -424,10 +425,15 @@ class ExternalProgram():
         if search_dir is None:
             return False
         trial = os.path.join(search_dir, name)
-        if not os.path.exists(trial):
+        if os.path.exists(trial):
+            if self._is_executable(trial):
+                return [trial]
+        else:
+            for ext in self.windows_exts:
+                trial_ext = '{}.{}'.format(trial, ext)
+                if os.path.exists(trial_ext):
+                    return [trial_ext]
             return False
-        if self._is_executable(trial):
-            return [trial]
         # Now getting desperate. Maybe it is a script file that is a) not chmodded
         # executable or b) we are on windows so they can't be directly executed.
         return self._shebang_to_cmd(trial)
@@ -441,6 +447,11 @@ class ExternalProgram():
         if fullpath or not mesonlib.is_windows():
             # On UNIX-like platforms, the standard PATH search is enough
             return [fullpath]
+        # On Windows, if name is an absolute path, we need the extension too
+        for ext in self.windows_exts:
+            fullpath = '{}.{}'.format(name, ext)
+            if os.path.exists(fullpath):
+                return [fullpath]
         # On Windows, interpreted scripts must have an extension otherwise they
         # cannot be found by a standard PATH search. So we do a custom search
         # where we manually search for a script with a shebang in PATH.