ccache + cotire 1.6.3

cotire 1.6.3 bugfix release:
* fixed bug where precompiled headers did not work with Clang (thanks to nh2 for
* reporting).
* when using ccache, require that environment variable CCACHE_SLOPPINESS is set
* to time_macros.
* 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.