Subzero: Add a makefile config for building with g++.

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> LIBCXX_INSTALL_PATH=<path> CLANG_PATH=<path> \
 #        PNACL_BIN_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 78 matching lines @@
 SB_CXX_DEFINES := $(BASE_CXX_DEFINES) -DPNACL_BROWSER_TRANSLATOR=1
 CXX_DEFINES := $(BASE_CXX_DEFINES) -DPNACL_BROWSER_TRANSLATOR=0
 
 ifdef NOASSERT
   ASSERTIONS = -DNDEBUG
 else
   ASSERTIONS =
   OBJDIR := $(OBJDIR)+Asserts
 endif
 
+# Use g++ to compile, to check for errors/warnings that clang++ might have
+# missed.  It's unlikely to link, unless LLVM was also built with g++, so the
+# compile_only target should be used.
+ifdef GPLUSPLUS
+  CXX = CCACHE_CPP2=yes $(CCACHE) g++
+  STDLIB_FLAGS =
+    LLVM_EXTRA_WARNINGS="-Wno-unknown-pragmas -Wno-unused-parameter \
+    -Wno-comment -Wno-enum-compare -Wno-strict-aliasing"
+  OBJDIR := $(OBJDIR)+Gplusplus
+endif
+
 ifdef UBSAN
   OBJDIR := $(OBJDIR)+UBSan
   CXX_EXTRA += -fsanitize=undefined -fno-sanitize=vptr \
                -fno-sanitize=nonnull-attribute
   LD_EXTRA += -fsanitize=undefined
 endif
 
 ifdef UBSAN_TRAP
   OBJDIR := $(OBJDIR)+UBSan_Trap
   CXX_EXTRA += -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error \
@@ skipping 321 matching lines @@
 check-spec: $(ALLSPEC:=.spec2k)
 
 check: check-lit check-unit check-xtest
 
 check-presubmit presubmit:
 # Make sure clang-format gets run.
         +make -f Makefile.standalone format
 # Verify MINIMAL build, plus proper usage of REQUIRES in lit tests.
         +make -f Makefile.standalone \
           MINIMAL=1 check
+# Check that there are no g++ build errors or warnings.
+        +make -f Makefile.standalone \
+          GPLUSPLUS=1 compile_only
 # Check the x86 assembler unit tests.
         +make -f Makefile.standalone \
           DEBUG=1 CHECK_X86_ASM=1 check-unit
-# Check that there are no g++ build errors or warnings.
-        +make -f Makefile.standalone clean
-        +make -f Makefile.standalone compile_only \
-          CXX=g++ STDLIB_FLAGS= \
-          LLVM_EXTRA_WARNINGS="-Wno-unknown-pragmas -Wno-unused-parameter \
-          -Wno-comment -Wno-enum-compare -Wno-strict-aliasing"
-        +make -f Makefile.standalone clean
 # Run spec2k for x86-32.
         +make -f Makefile.standalone \
           check check-spec
 # Build spec2k under -Om1/x86-32, to check for liveness errors.
         +make -f Makefile.standalone \
           SPECFLAGS='-Om1' SPECBUILDONLY=true check-spec
 # Run spec2k for x86-32 without advanced phi lowering.
         +make -f Makefile.standalone \
           SPECFLAGS='--sz=--phi-edge-split=0' check-spec
 # Build spec2k for arm32.
         +make -f Makefile.standalone \
           TARGET=arm32 SPECBUILDONLY=true check-spec
 # Build spec2k under -Om1/arm32.
         +make -f Makefile.standalone \
           TARGET=arm32 SPECFLAGS='-Om1' SPECBUILDONLY=true check-spec
 # Run a few spec2k tests for arm32 using qemu.
         +make -f Makefile.standalone \
           TARGET=arm32 ALLSPEC='176.gcc 181.mcf 254.gap' check-spec
-# Verify that all changes are committed (including clang-format above).
-        git diff --quiet
 # Provide validation of user awesomeness!
         echo Success
 
 FORMAT_BLACKLIST =
 # Add one of the following lines for each source file to ignore.
 FORMAT_BLACKLIST += ! -name IceParseInstsTest.cpp
 FORMAT_BLACKLIST += ! -name IceParseTypesTest.cpp
 FORMAT_BLACKLIST += ! -name assembler_arm.h
 FORMAT_BLACKLIST += ! -name assembler_arm.cc
 format:
@@ skipping 13 matching lines @@
 
 docs:
         doxygen Doxyfile
         @echo See file://`pwd`/docs/html/index.html
 
 clean:
         rm -rf pnacl-sz *.o $(OBJDIR) $(SB_OBJDIR) build/pnacl-sz.bloat.json
 
 clean-all: clean
         rm -rf build/ docs/