When the new RUNPATH validation phase was enabled by default, GCJ could
no longer be built.  After the GCJ build was fixed I noticed that
IcedTea6 failed because it could no longer find "gcj/libgcj-config.h".
I do not know what commit actually caused this failure.

A fix is attached.  The patch adds the target-dependent GCJ include
directory (whose "gcj" directory contains nothing more than
"libgcj-config.h") to the CPATH, so that the header file can be found at
build time.