bug#19733: disfunctional gcc binary when GCJ or gfortran is installed

[Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>]

Hi Guix,

installing the gcj or the gfortran package, my profile's /bin directory
gets a gcc link.  If I use Guix as a package manager on top of another
system that has a working installation of the GNU C compiler and my
profile bin path has preference to all other items in PATH, then I end
up with a disfunctional gcc binary.

gcj as well as gfortran come with a couple of binaries (gcc, gcc-ar,
etc) that do not constitute a working C compiler.  Trying to build an
application with

   ./configure
   make
   sudo make install

"configure" complains about gcc not being able to compile C code.

Other distributions seem to remove common binaries like gcc, gcc-ar, etc
from their gfortran packages and it seems that we should too in order to
avoid conflicts.  It seems that neither gfortran nor gcj actually need
these binaries to function.

Should we add another phase to the definitions of "custom-gcc" and "gcj"
to remove these binaries?

~~ Ricardo