build: turn on --enable-c++17 by --enable-fuzz

Fuzzing code uses C++17 specific code (e.g. std::optional), so it is not
possible to compile with --enable-fuzz and without --enable-c++17.

Thus, turn on --enable-c++17 whenever --enable-fuzz is used.

ci/test/00_setup_env_native_fuzz.sh

@@ -14,4 +14,4 @@ export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export RUN_FUZZ_TESTS=true
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer,address,undefined --enable-c++17 CC=clang CXX=clang++"
+export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=clang CXX=clang++"

ci/test/00_setup_env_native_fuzz_with_valgrind.sh

@@ -15,4 +15,4 @@ export RUN_FUNCTIONAL_TESTS=false
 export RUN_FUZZ_TESTS=true
 export FUZZ_TESTS_CONFIG="--valgrind"
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer --enable-c++17 CC=clang CXX=clang++"
+export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer CC=clang CXX=clang++"

configure.ac

@@ -69,7 +69,7 @@ AC_ARG_ENABLE([c++17],
   [use_cxx17=no])
 
 dnl Require C++11 or C++17 compiler (no GNU extensions)
-if test "x$use_cxx17" = xyes; then
+if test "x$use_cxx17" = xyes -o "x$enable_fuzz" = xyes ; then
   AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
 else
   AX_CXX_COMPILE_STDCXX([11], [noext], [mandatory])

doc/fuzzing.md

@@ -8,7 +8,7 @@ To quickly get started fuzzing Bitcoin Core using [libFuzzer](https://llvm.org/d
 $ git clone https://github.com/bitcoin/bitcoin
 $ cd bitcoin/
 $ ./autogen.sh
-$ CC=clang CXX=clang++ ./configure --enable-fuzz --with-sanitizers=address,fuzzer,undefined --enable-c++17
+$ CC=clang CXX=clang++ ./configure --enable-fuzz --with-sanitizers=address,fuzzer,undefined
 # macOS users: If you have problem with this step then make sure to read "macOS hints for
 # libFuzzer" on https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md#macos-hints-for-libfuzzer
 $ make
@@ -103,7 +103,7 @@ You may also need to take care of giving the correct path for `clang` and
 Full configure that was tested on macOS Catalina with `brew` installed `llvm`:
 
 ```sh
-./configure --enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=/usr/local/opt/llvm/bin/clang CXX=/usr/local/opt/llvm/bin/clang++ --disable-asm --enable-c++17
+./configure --enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=/usr/local/opt/llvm/bin/clang CXX=/usr/local/opt/llvm/bin/clang++ --disable-asm
 ```
 
 Read the [libFuzzer documentation](https://llvm.org/docs/LibFuzzer.html) for more information. This [libFuzzer tutorial](https://github.com/google/fuzzing/blob/master/tutorial/libFuzzerTutorial.md) might also be of interest.
@@ -121,7 +121,7 @@ $ git clone https://github.com/google/afl
 $ make -C afl/
 $ make -C afl/llvm_mode/
 $ ./autogen.sh
-$ CC=$(pwd)/afl/afl-clang-fast CXX=$(pwd)/afl/afl-clang-fast++ ./configure --enable-fuzz --enable-c++17
+$ CC=$(pwd)/afl/afl-clang-fast CXX=$(pwd)/afl/afl-clang-fast++ ./configure --enable-fuzz
 $ make
 # For macOS you may need to ignore x86 compilation checks when running "make". If so,
 # try compiling using: AFL_NO_X86=1 make
@@ -148,7 +148,7 @@ $ git clone https://github.com/google/honggfuzz
 $ cd honggfuzz/
 $ make
 $ cd ..
-$ CC=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang CXX=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang++ ./configure --enable-fuzz --with-sanitizers=address,undefined --enable-c++17
+$ CC=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang CXX=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang++ ./configure --enable-fuzz --with-sanitizers=address,undefined
 $ make
 $ mkdir -p inputs/
 $ honggfuzz/honggfuzz -i inputs/ -- src/test/fuzz/process_message