strnlen not defined in Cygwin.

[rianquinn]

If you try to use the GSL in cygwin, you get:

"error: 'strnlen' was not declared in this scope".

Looking this up, it appears that strnlen is not a standard function. If this is true, we should either remove it, or provide our own implementation. Note that the link below is for MinGW but I am seeing this with GCC 5.4 in cygwin as well.

http://stackoverflow.com/questions/35197129/mingw-g-strnlen-was-not-declared-in-this-scope

[neilmacintosh]

@rianquinn this is a duplicate of #315, so I'll close it in favor of that issue.