#53 Fix building on GNU/Hurd

[Pino Toscano]

On GNU/Hurd the POSIX-optional PATH_MAX is not defined on purpose, and neither is the non-standard MAXPATHLEN. This causes build failures in three places.

Attached there are few patches to replace them with
- usage of POSIX.1-8 behaviour of realpath (NULL as second argument returns a newly allocated buffer for the result)
- dynamic memory allocation for the strings

[Pino Toscano]

Patch for removing PATH_MAX usage (excluding the code paths for realpath not supporting the newer POSIX behaviour).

[Pino Toscano]

Patch for removing non-standard MAXPATHLEN usage.
Most probably the sys/param.h could be removed, but I did not test that.

[Pino Toscano]

Ah, forgot to say the patches have been done on the latest mjpegtools 2.1.0.

[Bernhard Praschinger]

Seems that the patch does not break anything, so I did put it into the CVS it will appear in the next release.