patchMakefile: use CXXFLAGS for linking fuzzers

>
> OSS-Fuzz requires C++-specific flags to link fuzzers. Passing these in
> CFLAGS causes lots of build warnings. Using separate CXXFLAGS avoids
> this.
>

>  LDFLAGS =
>  ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
>  ALL_LDFLAGS = $(LDFLAGS)
> @@ -3098,14 +3099,14 @@ cover_db_html: cover_db
>  # An example command to build against libFuzzer from LLVM 4.0.0:
>  #
>  # make CC=clang CXX=clang++ \
> -#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
> +#      CXXFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
>  #      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
>  #      fuzz-all
>  #
>  .PHONY: fuzz-all

>
>  $(FUZZ_PROGRAMS): all
> -       $(QUIET_LINK)$(CXX) $(CFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
> +       $(QUIET_LINK)$(CXX) $(CXXFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
>                 $(XDIFF_OBJS) $(EXTLIBS) git.o $@.o $(LIB_FUZZING_ENGINE) -o $@

> Signed-off-by: Josh Steadmon <steadmon@google.com>
> ---
>  Makefile | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index bbfbb4292d..5462bc4b6b 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -497,6 +497,7 @@ GIT-VERSION-FILE: FORCE
>  # CFLAGS and LDFLAGS are for the users to override from the command line.
>  
>  CFLAGS = -g -O2 -Wall
> +CXXFLAGS ?= $(CFLAGS)
>  LDFLAGS =
>  ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
>  ALL_LDFLAGS = $(LDFLAGS)
> @@ -3098,14 +3099,14 @@ cover_db_html: cover_db
>  # An example command to build against libFuzzer from LLVM 4.0.0:
>  #
>  # make CC=clang CXX=clang++ \
> -#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
> +#      CXXFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
>  #      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
>  #      fuzz-all
>  #
>  .PHONY: fuzz-all
>  
>  $(FUZZ_PROGRAMS): all
> -	$(QUIET_LINK)$(CXX) $(CFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
> +	$(QUIET_LINK)$(CXX) $(CXXFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
>  		$(XDIFF_OBJS) $(EXTLIBS) git.o $@.o $(LIB_FUZZING_ENGINE) -o $@
>  
>  fuzz-all: $(FUZZ_PROGRAMS)

> steadmon@google.com writes:
> 
> > OSS-Fuzz requires C++-specific flags to link fuzzers. Passing these in
> > CFLAGS causes lots of build warnings. Using separate CXXFLAGS avoids
> > this.
> 
> We are not a C++ shop, so allow me to show ignorance about how
> projects that are OSS-Fuzz-enabled work.  Do they use one set of
> CXXFLAGS when compiling the "real thing" and a separate set (perhaps
> one is subset of the other, or perhaps these two just have overlap)
> of CXXFLAGS when building to link with the fuzzer?
> 
> What I am trying to get at is if this should be CXXFLAGS or
> CXX_FUZZER_FLAGS.  If the OSS-Fuzz-enabled C++ projects use one set
> of flags for the "main" part of the project (to produce binaries to
> be run by the end users) and then link with extra flags to work with
> fuzzers, I would imagine that they won't call the latter CXXFLAGS
> but call it something else, and we probably should follow suit if
> that is the case.
> 
> Not that we plan to (re)write the maint part of Git in C++ ever, so
> I am personally OK with sacrificing the most generic CXXFLAGS macro
> for the sole use of OSS-Fuzz linkage, but I'd prefer to leave the
> door open so that other things like OSS-Fuzz that require C++ can be
> added like your work does to the project.
> 
> Thanks.