--with-gmp and --with-libgmp-prefix

Hi Joel,

I was talking to a colleague and we noticed the top-level binutils-gdb 
configure accepts --with-gmp, which looks like what gcc's configure uses 
to specify the gmp library installation path.

But gdb's configure expects that information to be passed through the 
--with-libgmp-prefix switch. Was that a deliberate choice (I'm looking 
at commit 2c947d9bc2ff3b84f1676b83577fe1bba05b4a5f).

When one is used to building gcc, it is a little odd to have to specify 
--with-libgmp-prefix instead of --with-gmp.

The top-level configure lists this:

   --with-gmp=PATH         specify prefix directory for the installed GMP
                           package. Equivalent to
                           --with-gmp-include=PATH/include plus
                           --with-gmp-lib=PATH/lib

And gdb's configure lists this:

   --with-libgmp-prefix[=DIR]  search for libgmp in DIR/include and DIR/lib

I'm wondering if we can reuse the top-level configure's --with-gmp option.