[GCC-XML] make install broken

[Neal Becker]

I grabbed a brand-new cvs of gccxml, and on linux did:

1. cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr
2. make
3. sudo make install

And where did it install?
-- Installing /bin/gccxml
-- Installing /share/man/man1/gccxml.1
...

What happened to my /usr prefix?

Also, make install did:
Linking C executable CMakeFiles/CMakeRelink.dir/gccxml_cc1plus
Linking CXX executable CMakeFiles/CMakeRelink.dir/gccxml

That's not nice.  Make install should never write to the build directory, 
otherwise sudo make install won't work through nfs.