Subzero: Use ccache for building, if available.

Makefile.standalone

 # The following variables will likely need to be modified, depending on where
 # and how you built LLVM & Clang. They can be overridden in a command-line
 # invocation of make, like:
 #
 #   make LLVM_SRC_PATH=<path> LLVM_BIN_PATH=<path> \
 #        LIBCXX_INSTALL_PATH=<path> CLANG_PATH=<path> ...
 #
 
 # LLVM_SRC_PATH is the path to the root of the checked out source code. This
 # directory should contain the configure script, the include/ and lib/
@@ skipping 33 matching lines @@
 $(info Using CLANG_PATH = $(CLANG_PATH))
 $(info Using HOST_ARCH     = $(HOST_ARCH))
 $(info -----------------------------------------------)
 
 LLVM_CXXFLAGS := `$(LLVM_BIN_PATH)/llvm-config --cxxflags`
 LLVM_LDFLAGS := `$(LLVM_BIN_PATH)/llvm-config --libs` \
                 `$(LLVM_BIN_PATH)/llvm-config --ldflags`
 
 # It's recommended that CXX matches the compiler you used to build LLVM itself.
 OPTLEVEL := -O0
-CXX := $(CLANG_PATH)/clang++
+CCACHE := `command -v ccache`
+CXX := CCACHE_CPP2=yes $(CCACHE) $(CLANG_PATH)/clang++
 
 CXXFLAGS := $(LLVM_CXXFLAGS) -Wall -Wextra -Werror -fno-rtti \
         -fno-exceptions $(OPTLEVEL) -g $(HOST_FLAGS) \
         -Wno-error=unused-parameter -I$(LIBCXX_INSTALL_PATH)/include/c++/v1
 LDFLAGS := $(HOST_FLAGS) -L$(LIBCXX_INSTALL_PATH)/lib
 
 SRCS= \
         IceCfg.cpp \
         IceCfgNode.cpp \
         IceConverter.cpp \
@@ skipping 54 matching lines @@
   CLANG_FORMAT_DIFF = clang-format-diff.py
 else
   CLANG_FORMAT_DIFF = /usr/lib/clang-format/clang-format-diff.py
 endif
 format-diff:
         git diff -U0 `git merge-base HEAD master` | \
         $(CLANG_FORMAT_DIFF) -p1 -style=LLVM -i
 
 clean:
         rm -rf llvm2ice *.o build/