Merge "ccache: Allow external setting of CCACHE_COMPILERCHECK variable"

ifneq ($(filter-out false,$(USE_CCACHE)),)

  # The default check uses size and modification time, causing false misses

  # since the mtime depends when the repo was checked out

  export CCACHE_COMPILERCHECK := content

  export CCACHE_COMPILERCHECK ?= content

  # See man page, optimizations to get more cache hits

  # implies that __DATE__ and __TIME__ are not critical for functionality.