cotire 1.6.3

Bugfix release:
* don't use -w flag for pre-compiling the prefix header, because it has
unwanted side effects.
* correctly handle linked targets' INTERFACE_COMPILE_OPTIONS,
INTERFACE_INCLUDE_DIRECTORIES
  and INTERFACE_COMPILE_DEFINITIONS properties upon pre-compiling and
prefix header generation.
* For Clang and GCC, pre-compile prefix header through indirect
inclusion via a prefix source file,
  to make both compilers honor the system_header pragma in the prefix
header correctly.
* fix ccache incompatibility.
* correctly handle usage requirements for transitive link targets.
* use indirect inclusion for prefix header when using generator Xcode.