Add command line option to CMake's options

It solves the issues when one generated makefiles with
some of the custom command line options passed to CMake
and then modified the CMake file. This used to forget
about settings passed via command line.

Now it's no longer required to re-run CMake with all
custom options every time CMakeLists.txt files are
modified.
Add an option to disable OpenCL

It's not always wanted to compile OpenSubdiv with OpenCL
support when the SDK is installed on the builder machine.
Don't show warnings when components are disabled
Document NO_MAYA command line option

If was used in CMake but was never documented.
Make PTex support optional

Basically makes configuration log much cleaner on systems
where PTex is not installed.
Fix compilation error on windows+msvc2013

- Some missing includes of <algorithms> in order to have
  stdd::min() and similar functions.
- Need to cast numIndices and numNVerts to int explicitly
  in order to solve warning treated as an error about
  precision loss.
- Can't do vector[0] for an empty vector, it'll generate
  a runtime range check error.
- MSVC only works fine with make_pair(foo, bar) syntax,
  without explicit template substitution here. Otherwise
  weird 'can't cast int to int&&' errors are happening.