libscrypt fail on Wimplicit-fallthrough

[1am]

#Hi

I was trying to build cpp-ethereum but it fails on the following part:

$ mkdir build && cd build
$ cmake ..
...
-- The C compiler identification is GNU 5.4.1
-- The CXX compiler identification is GNU 7.2.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Performing Test fstack-protector-strong
-- Performing Test fstack-protector-strong - Success
-- Performing Test Wimplicit-fallthrough
-- Performing Test Wimplicit-fallthrough - Success
...

$ cmake --build .
...
Scanning dependencies of target devcore
[ 10%] Building CXX object libdevcore/CMakeFiles/devcore.dir/Address.cpp.o
[ 10%] Building CXX object libdevcore/CMakeFiles/devcore.dir/Base64.cpp.o
[ 11%] Building CXX object libdevcore/CMakeFiles/devcore.dir/Common.cpp.o
[ 11%] Building CXX object libdevcore/CMakeFiles/devcore.dir/CommonData.cpp.o
[ 12%] Building CXX object libdevcore/CMakeFiles/devcore.dir/CommonIO.cpp.o
[ 12%] Building CXX object libdevcore/CMakeFiles/devcore.dir/CommonJS.cpp.o
[ 13%] Building CXX object libdevcore/CMakeFiles/devcore.dir/FileSystem.cpp.o
[ 13%] Building CXX object libdevcore/CMakeFiles/devcore.dir/FixedHash.cpp.o
[ 13%] Building CXX object libdevcore/CMakeFiles/devcore.dir/Guards.cpp.o
[ 14%] Building CXX object libdevcore/CMakeFiles/devcore.dir/JsonUtils.cpp.o
[ 14%] Building CXX object libdevcore/CMakeFiles/devcore.dir/Log.cpp.o
[ 15%] Building CXX object libdevcore/CMakeFiles/devcore.dir/MemoryDB.cpp.o
[ 15%] Building CXX object libdevcore/CMakeFiles/devcore.dir/OverlayDB.cpp.o
[ 16%] Building CXX object libdevcore/CMakeFiles/devcore.dir/RLP.cpp.o
[ 16%] Building CXX object libdevcore/CMakeFiles/devcore.dir/SHA3.cpp.o
[ 16%] Building CXX object libdevcore/CMakeFiles/devcore.dir/TransientDirectory.cpp.o
[ 17%] Building CXX object libdevcore/CMakeFiles/devcore.dir/TrieCommon.cpp.o
[ 17%] Building CXX object libdevcore/CMakeFiles/devcore.dir/TrieHash.cpp.o
[ 18%] Building CXX object libdevcore/CMakeFiles/devcore.dir/Worker.cpp.o
[ 18%] Linking CXX static library libdevcore.a
[ 18%] Built target devcore
Scanning dependencies of target scrypt
[ 19%] Building C object utils/libscrypt/CMakeFiles/scrypt.dir/b64.c.o
cc: error: unrecognized command line option '-Wimplicit-fallthrough'
utils/libscrypt/CMakeFiles/scrypt.dir/build.make:62: recipe for target 'utils/libscrypt/CMakeFiles/scrypt.dir/b64.c.o' failed
make[2]: *** [utils/libscrypt/CMakeFiles/scrypt.dir/b64.c.o] Error 1
CMakeFiles/Makefile2:1248: recipe for target 'utils/libscrypt/CMakeFiles/scrypt.dir/all' failed
make[1]: *** [utils/libscrypt/CMakeFiles/scrypt.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2

Im trying to build on Ubuntu 17.10 with cmake 3.9.1.
Could you tell me where to look for ways to fix this issue?

[chfast]

Fixed by #4873