nan: Relicense under LGPLv2+

[Bruno Haible]

The module 'nan' (previously tests/nan.h, now lib/nan.h) defines functions
for getting a NaN value reliably. This module is, so far, under GPL,
because historically only the tests use it.

I would like to use it under LGPLv2+, for the modules 'setpayload*'
and 'getpayloadl' (functions defined by ISO C 23 and implemented in glibc).

Can I have your agreement to relicensing your contributions to this
module under LGPLv2+, please?

  * Paul Eggert:
    
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=16f6a8d8d81cc93745a24c0fb89caab2c383ae3c#patch7
  * Eric Blake:
    
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=ecab4471c7da5d367a62c8db75ee54ed6d948e40#patch11
  * Daniel Richard G.:
    
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=74e641c2e51562304bebbd69bbe0919be027ccde#patch9

Bruno