CMake: Better ccache configuration

EthCcache module taken from cpp-ethereum.

cmake/EthCcache.cmake

@@ -0,0 +1,15 @@
+# Setup ccache.
+#
+# The ccache is auto-enabled if the tool is found.
+# To disable set -DCCACHE=OFF option.
+if(NOT DEFINED CMAKE_CXX_COMPILER_LAUNCHER)
+  find_program(CCACHE ccache DOC "ccache tool path; set to OFF to disable")
+  if(CCACHE)
+    set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE})
+    if(COMMAND cotire)
+      # Change ccache config to meet cotire requirements.
+      set(ENV{CCACHE_SLOPPINESS} pch_defines,time_macros)
+    endif()
+    message(STATUS "[ccache] Enabled: ${CCACHE}")
+  endif()
+endif()