[patch] find libgcj.so when version of gcc != version of gcj

[Lillian Angel]

Matthias Klose wrote:
> The build system assumes that libgcj.so can be found when using gcc, which is
> not the case for different gcc and gcj versions. This adds an additional path to
> search for. It allows a user to use GCJ (unset by default), or else replaces gcc
> with gcj, and calls gcj --print-file-name=libgcj.so.
> 
> ok to checkin?

fine with me.