Make libf2c a shared library

[David Edelsohn <dje at watson dot ibm dot com>]

	Now that building libf2c creates a shared library, the build on
AIX fails because the link command for one of the multilibs overflows the
OS argument list.

	Would it be possible to use a build process similar to
libstdc++-v3 which uses intermediate archives (libsupc++convenience.a and
libmath.a)?  In other words, create an archive of the object files in
libI77, libF77, libU77, and libE77; then create a shared libg2c.so library
from the archives instead of explicitly listing all object files.

	Listing all of the objects on the link line is not portable,
especially for _POSIX_ARG_MAX of 4096.  AIX is not limited to that value,
but your Makefile for building the shared libf2c library (further) breaks
POSIX portability of the GCC build process.

Thanks, David