[x265] Building on Cygwin

[Phil Rosenberg]

> I expect a cygwin build to be slower than either a MinGW or MSVC build.

I can't profess to be an expert here, but my understanding is that
Cygwin is just a dll and a collection of utilities. The dll converts
Linux system calls to Windows system calls, but otherwise any code
that you compile should be no slower than natively compiled code.
Unless system calls are a significant bottleneck I imagine the
difference will be minimal.
The advantage of Cygwin when building open source code is having
access to a package manager and Linux-like environment. So in my case
when building ffmpeg with a number of additional dependencies I could
install most dependencies with the package manager and easily build
the rest then build ffmpeg using standard the standard ./configure,
make, make install. This was an order of magnitude easier than trying
to build via visual studio (which I ordinarily prefer) and
significantly easier than having to build all the dependancies (and
their dependancies...) with MinGW.
Mostly a library's Linux build system will just work on Cygwin. This
is one of the rare cases that doesn't. Here the -znoexecstack option
isn't valid as it relates to ELF executables which we aren't building.
For the sake of a single line of code to avoid that error  I would
implore you to simply include it and forget about it. The other
warning doesn't even really apply, it is just generated for every
Cygwin build which requires a CMake version before a certain date due
to non-backwards-compatible change. x265 clearly wasn't affected by
that change, so you can choose to either ignore it or silence it.

Again, it would be really useful if you would include that one line
change. It is unlikely that you will ever have to worry about Cygwin
compatibility again so I can't see why you wouldn't.

Phil