dependencies: Handle /usr/bin/env shebangs on Haiku

/usr/bin/env does not exist on Haiku since there's no /usr. The actual
location is /bin/env. Detect that case and directly use the
interpreter being passed to `env` in the shebang.

Also reorganize the Windows special cases which does the same thing.

mesonbuild/dependencies/base.py

@@ -626,9 +626,20 @@ class ExternalProgram:
                     # Windows does not ship python3.exe, but we know the path to it
                     if len(commands) > 0 and commands[0] == 'python3':
                         commands = mesonlib.python_command + commands[1:]
+                elif mesonlib.is_haiku():
+                    # Haiku does not have /usr, but a lot of scripts assume that
+                    # /usr/bin/env always exists. Detect that case and run the
+                    # script with the interpreter after it.
+                    if commands[0] == '/usr/bin/env':
+                        commands = commands[1:]
+                    # We know what python3 is, we're running on it
+                    if len(commands) > 0 and commands[0] == 'python3':
+                        commands = mesonlib.python_command + commands[1:]
                 return commands + [script]
-        except Exception:
+        except Exception as e:
+            mlog.debug(e)
             pass
+        mlog.debug('Unusable script {!r}'.format(script))
         return False
 
     def _is_executable(self, path):
@@ -659,21 +670,17 @@ class ExternalProgram:
                         return [trial_ext]
         return False
 
-    def _search(self, name, search_dir):
+    def _search_windows_special_cases(self, name, command):
         '''
-        Search in the specified dir for the specified executable by name
-        and if not found search in PATH
+        Lots of weird Windows quirks:
+        1. PATH search for @name returns files with extensions from PATHEXT,
+           but only self.windows_exts are executable without an interpreter.
+        2. @name might be an absolute path to an executable, but without the
+           extension. This works inside MinGW so people use it a lot.
+        3. The script is specified without an extension, in which case we have
+           to manually search in PATH.
+        4. More special-casing for the shebang inside the script.
         '''
-        commands = self._search_dir(name, search_dir)
-        if commands:
-            return commands
-        # Do a standard search in PATH
-        command = shutil.which(name)
-        if not mesonlib.is_windows():
-            # On UNIX-like platforms, shutil.which() is enough to find
-            # all executables whether in PATH or with an absolute path
-            return [command]
-        # HERE BEGINS THE TERROR OF WINDOWS
         if command:
             # On Windows, even if the PATH search returned a full path, we can't be
             # sure that it can be run directly if it's not a native executable.
@@ -687,7 +694,7 @@ class ExternalProgram:
             commands = self._shebang_to_cmd(command)
             if commands:
                 return commands
-        else:
+            return [None]
         # Maybe the name is an absolute path to a native Windows
         # executable, but without the extension. This is technically wrong,
         # but many people do it because it works in the MinGW shell.
@@ -706,6 +713,22 @@ class ExternalProgram:
                 return commands
         return [None]
 
+    def _search(self, name, search_dir):
+        '''
+        Search in the specified dir for the specified executable by name
+        and if not found search in PATH
+        '''
+        commands = self._search_dir(name, search_dir)
+        if commands:
+            return commands
+        # Do a standard search in PATH
+        command = shutil.which(name)
+        if mesonlib.is_windows():
+            return self._search_windows_special_cases(name, command)
+        # On UNIX-like platforms, shutil.which() is enough to find
+        # all executables whether in PATH or with an absolute path
+        return [command]
+
     def found(self):
         return self.command[0] is not None