fixed dependency of stitching module on xfeatures2d module as optional
fixed the initModule_xfeatures2d function that was called in module stitching since it is in another namespace than the standard cv one.

This reverts commit fba72cb6, reversing
changes made to 02131ffb.

added optional dependency stitching->nonfree. When nonfree is available, make sure linker does not throw it away.

 [~] Automatically tracked dependencies between modules
 [+] Support for optional module dependencies
 [+] Options to choose modules to build
 [~] Removed hardcoded modules lists from OpenCVConfig.cmake, opencv.pc and OpenCV.mk
 [+] Added COMPONENTS support for FIND_PACKAGE(OpenCV)
 [~] haartraining and traincascade are moved outside of modules folder since they aren't the modules

renamed OPENCV_SOURCE_DIR to OpenCV_SOURCE_DIR

[~] fixed warning in featuers2d under windows (can't generate assignment operator)

Added option to force disable precompiled headers.