Pier Luigi,
We are properly using FiveMac in Leopard.
You don't need to specify the path for gcc:
/Xcode2.5/usr/bin/gcc
Also, you don't need to set this headers path for gcc:
-I/Xcode2.5/SDKs/MacOSX10.4u/usr/include
We email you the most recent FiveMac build for Leopard