Guarantee a definition of NAN.

Guarantee a definition of NAN.

* lib/math.in.h (NAN): Define if missing.
* tests/test-math.c (main): Test it.
* doc/posix-headers/math.texi (math.h): Document this.
* lib/isnan.c (rpl_isnand): Use it.
* tests/test-ceilf1.c (NaN): Delete, and use NAN instead.
* tests/test-floorf1.c (NaN): Likewise.
* tests/test-frexp.c (NaN): Likewise.
* tests/test-isnand.c (NaN): Likewise.
* tests/test-isnanf.c (NaN): Likewise.
* tests/test-round1.c (NaN): Likewise.
* tests/test-roundf1.c (NaN): Likewise.
* tests/test-snprintf-posix.h (NaN): Likewise.
* tests/test-sprintf-posix.h (NaN): Likewise.
* tests/test-trunc1.c (NaN): Likewise.
* tests/test-truncf1.c (NaN): Likewise.
* tests/test-vasnprintf-posix.c (NaN): Likewise.
* tests/test-vasprintf-posix.c (NaN): Likewise.
* modules/isnand-nolibm (Depends-on): Add math.
* modules/isnanf-nolibm (Depends-on): Likewise.
* modules/isnanl (Depends-on): Likewise.
* modules/isnanl-nolibm (Depends-on): Likewise.
* modules/snprintf-posix-tests (Depends-on): Likewise.
* modules/sprintf-posix-tests (Depends-on): Likewise.
* modules/vsnprintf-posix-tests (Depends-on): Likewise.
* modules/vsprintf-posix-tests (Depends-on): Likewise.
* modules/vasnprintf-posix-tests (Depends-on): Likewise.
* modules/vasprintf-posix-tests (Depends-on): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>