No, this won't help at all: even in that case, GCC won't provide a
__builtin_isnan. See tm.texi for TARGET_C99_FUNCTIONS:
@defmac TARGET_C99_FUNCTIONS
When this macro is nonzero, GCC will implicitly optimize @code{sin}
calls into
@code{sinf} and similarly for other functions defined by C99 standard.
The
default is nonzero that should be proper value for most modern
systems, however
number of existing systems lacks support for these functions in the
runtime so
they needs this macro to be redefined to 0.
@end defmac
But otherwise you're right: if Solaris 10 provides a complete ISO C99
library which is usable by GCC, then we should have e.g. sol10.h which
defines that macro.