mruby-math: remove Cygwin compatibility macros

nan:
 The first non-test release of Cygwin 1.5.x was over a decade ago:
 <http://cygwin.com/ml/cygwin-announce/2003-09/msg00001.html>

log/log10:
 This workaround is not needed anymore (1.2.6.1, 1.3 - both over 6 years ago):
 <https://sourceware.org/cgi-bin/cvsweb.cgi/src/newlib/libm/math/w_log.c?cvsroot=src>