Enable libdng_sdk for Windows

This is needed to use skia in Android Studio for Windows.
Changes include:
  - Replacing in dng_pthread.cpp '&operator(...) { }' with '&operator(...);'
to prevent 'control reaches end of non-void function' errors
  - Removing test of '_MSC_VER >= 1900' in dng_pthread.h as we use clang
for compilation which has a recent version of 'time.h' but fails the
test '_MSC_VER >= 1900'

Test: sdk build
Change-Id: I200d691378c5d3e7c1729b5f645766755aa3e296
3 files changed