Support ternary --with-qd

[orlitzky]

I'm packaging fplll for Gentoo, and the --with-qd flag is giving me some trouble. Right now the flag supports two things:

• Disabling the support via --without-qd. This works OK.
• Enabling the support via --with-qd=PATH. This only sort-of works.

The main problem with --with-qd=PATH is that there's no way for me to set the path to the right place if the libraries are in e.g. /usr/lib64 and the headers in /usr/include. If /usr/lib points to 32-bit libraries on a multilib system, passing --with-qd=/usr will try to link those 32-bit libraries into a 64-bit fplll. No bueno.

On the other hand, if I don't supply the path, then the bare --with-qd sets the path to the literal value "yes". And that doesn't work, because there are no libraries in yes/lib.

Would it be possible to add another case for x$with_qd = "xyes", that will use the standard system locations for the library and headers? That would avoid the first problem in my case: if you do an AC_SEARCH_LIBS on a 64 bit system, it should only find 64 bit libs.

[malb]

I agree, this should be done. Any chance you could provide a PR? Otherwise, I'll try to get around to it when I can.