Avoid warning about missing getpagesize() prototype on Windows
Work-around AC_CHECK_FUNCS([getpagesize]) reporting a false positive when using MinGW gcc, due to it being present in libgcc.a, which results in trying to use it without a prototype.
Future work: This value control the default memory allocation size for text widgets. If there's some reason why a fallback to using BUFSIZ is bad, we could use the actual pagesize instead.