I believe this distribution package has no permission to be writable by normal users (normal, but not something a non-experienced user would expect). chmod-ing the whole directory tree to 666 (for files) and 777 (for directories) should fix it.
@Leledumbo: Changing permission doesn't help as the makefile.fpc itself is missing.
With this solution bellow I was able to "make" the cross compiler although I was not able to verify the result:
1) delete the whole /usr/share/fpcsrc/3.0.0 directory tree
2) download and unpack fpc-3.0.0.source.tar.gz from freepascal.org in /usr/share/fpcsrc/
3) rename the created /usr/share/fpcsrc/fpc-3.0.0 directory to /usr/share/fpcsrc/3.0.0
4) now fpc compilation will be OK:
cd /usr/share/fpcsrc/3.0.0
sudo make all CPU_TARGET=i386
sudo make crossinstall CPU_TARGET=i386
5) the new fp compiler is /usr/local/lib/fpc/3.0.0/ppcross386 (I think), make a link to it:
cd /usr/bin
sudo ln --symbolic -T /usr/local/lib/fpc/3.0.0/ppcross386 ./ppc386
Unfortunately the result was such a different and chaotic directory tree where my Lazarus project was not able to find the external references. I tried to modify it step by step but at the end I gave it up. An additional reason of fail can also be that my original source package was developed with fpc-2.4 and it seems so that there were a lot of changes also in the libraries.
Anyhow thanks for all of your help!