isnan: Fix porting problem to Solaris 10 with bundled gcc.

isnan: Fix porting problem to Solaris 10 with bundled gcc.

Without this fix, the command to link temacs failed due to an
undefined symbol __builtin_isnan.  This is because
/usr/include/iso/math_c99.h #defines isnan(x) to
__builtin_isnan(x), but the bundled gcc, which identifies itself
as gcc 3.4.3 (csl-sol210-3_4-branch+sol_rpath), does not have
a __builtin_isnan.
* configure.in (isnan): Remove now-unnecessary check.
* src/floatfns.c (isnan): #undef, and then #define to a clone of
what's in data.c.
(Fisnan): Always define, since it's always available now.
(syms_of_floatfns): Always define isnan at the Lisp level.