Fix erroneous USE_LONG_DOUBLE in configure.com.

Fix erroneous USE_LONG_DOUBLE in configure.com.

Once upon a time there was a "use64bit" option [1] that only later
became separated into use64bitint, uselongdouble, and use64bitall,
but we didn't properly separate out everything.  So if you chose
64-bit integers but not long doubles, you would get the macro
USE_LONG_DOUBLE defined but without other supporting defines and
with incompatible branches followed in various parts of the #ifdef
jungle.  So separate them out.  Thanks to Thomas Pfau for trying
what's apparently a rare configuration.

[1] See fafa4fee6354847ae7fda.