libfortran: Fix execute_command_line for Windows

On Windows, 'system' is called - that fails with -1 if the command
interpreter could not be started; on POSIX systems, if the child
process could not be started by the shell, exit(127)/_exit(127) is
called/returned. On Windows, cmd.exe (and also the PowerShell) return
errorlevel 9009.

libgfortran/ChangeLog:

	* intrinsics/execute_command_line.c (execute_command_line): On
	Windows, regard system()'s return value of 9009 as EXEC_INVALIDCOMMAND.

diff --git a/libgfortran/intrinsics/execute_command_line.c b/libgfortran/intrinsics/execute_command_line.c
index 305f067d973..0d1688400c2 100644
--- a/libgfortran/intrinsics/execute_command_line.c
+++ b/libgfortran/intrinsics/execute_command_line.c
@@ -142,10 +142,15 @@ execute_command_line (const char *command, bool wait, int *exitstat,
 #endif
       else if (res == 127 || res == 126
 #if defined(WEXITSTATUS) && defined(WIFEXITED)
 	       || (WIFEXITED(res) && WEXITSTATUS(res) == 127)
 	       || (WIFEXITED(res) && WEXITSTATUS(res) == 126)
+#endif
+#ifdef __MINGW32__
+		  /* cmd.exe sets the errorlevel to 9009,
+		     if the command could not be executed.  */
+		|| res == 9009
 #endif
 	       )
 	/* Shell return codes 126 and 127 mean that the command line could
 	   not be executed for various reasons.  */
 	set_cmdstat (cmdstat, EXEC_INVALIDCOMMAND);