Panther (10.3.3) 使ってるけど同じエラー出た。
$ gcc isnan.cpp -o isnan
isnan.cpp: In function `int main()':
isnan.cpp:2: error: `isnan' undeclared (first use this function)
isnan.cpp:2: error: (Each undeclared identifier is reported only once for each
function it appears in.)
$ gcc -v
Reading specs from /usr/libexec/gcc/darwin/ppc/3.3/specs
Thread model: posix
gcc version 3.3 20030304 (Apple Computer, Inc. build 1495)