Fix the g++ build.

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 76 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 := $(OBJDIR)+UBSan_Trap
   CXX_EXTRA += -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error \
@@ skipping 67 matching lines @@
                    -L$(SB_LLVM_PATH)/lib
 
 CCACHE := `command -v ccache`
 CXX := CCACHE_CPP2=yes $(CCACHE) $(CLANG_PATH)/clang++
 SB_CXX := CCACHE_CPP2=yes $(CCACHE) $(PNACL_BIN_PATH)/pnacl-clang++
 SB_TRANSLATE := $(PNACL_BIN_PATH)/pnacl-translate
 
 # Extra warnings that LLVM's build system adds in addition to -Wall.
 LLVM_EXTRA_WARNINGS := -Wcovered-switch-default
 
+# 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.  Note: This ifdef section is deliberately
+# placed here instead of with the other ifdef sections, so that its redefinition
+# of CXX/STDLIB_FLAGS/LLVM_EXTRA_WARNINGS follows their normal definitions.
+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 \
+                         -Wno-return-type
+  OBJDIR := $(OBJDIR)+Gplusplus
+endif
+
 BASE_CXXFLAGS := -std=gnu++11 -Wall -Wextra -Werror -fno-rtti \
   -fno-exceptions $(OPTLEVEL) $(ASSERTIONS) -g -pedantic \
   $(LLVM_EXTRA_WARNINGS) $(CXX_EXTRA)
 
 CXXFLAGS := $(LLVM_CXXFLAGS) $(BASE_CXXFLAGS) $(CXX_DEFINES) $(HOST_FLAGS) \
   $(STDLIB_FLAGS)
 SB_CXXFLAGS := $(SB_LLVM_CXXFLAGS) $(BASE_CXXFLAGS) $(SB_CXX_DEFINES)
 
 LDFLAGS := $(HOST_FLAGS) -L$(LIBCXX_INSTALL_PATH)/lib -Wl,--gc-sections \
   $(LD_EXTRA) $(STDLIB_FLAGS)
@@ skipping 305 matching lines @@
         @echo See Subzero size breakdown in bloat/pnacl-sz.bloat.html
 
 docs:
         make -C docs -f Makefile.standalone
 
 clean:
         rm -rf pnacl-sz *.o $(OBJDIR) $(SB_OBJDIR) build/pnacl-sz.bloat.json
 
 clean-all: clean
         rm -rf build/  crosstest/Output/