build error on cygwin fixed

[Yakov Lerner]

Hello,
I just downloaded and built gnu m4 on cygwin.
However, I had to patch m4.c to avoid following error:

  gcc -c -DHAVE_CONFIG_H -I.. -I. -I./../lib  -g -O m4.c
  m4.c: In function `main':
  m4.c:447: `SIG_IGN' undeclared (first use in this function)
  m4.c:447: (Each undeclared identifier is reported only once

The fix is to add '#include <signal.h>' to m4.c
Apparently, in cygwin, '<sys/signal.h>' is not the same as 'signal.h'

Regards

Jacob Lerner