ccache + cotire 1.6.3

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.