Fix boost::system detection in case of using clang-3.8
Fix boost::system detection in case of using clang-3.8

Trying to compile the project using clang-3.8 with e.g. the following
configure options on a Ubuntu 64 bit system, 18.04 and libboost version 1.65:

```./configure --disable-wallet --disable-bench --with-gui=no CC=clang-3.8 CXX=clang++-3.8 CXXFLAGS=-std=c++14```

will fail with something like this:

```
checking for mismatched boost c++11 scoped enums... ok
configure: error: No working boost sleep implementation found.
Makefile:602: recipe for target 'config.status' failed
make: *** [config.status] Error 1
```

The actual reason is that boost::system is not properly detected because
the additional CXXFLAGS argument of '-std=c++14' is not passed to clang-3.8
in the ax_boost_system.m4 autoconf detection macro, but rather the value
of CXXFLAGS is cleared before clang-3.8 is invoked. Without this flag,
compilation of the test case in ax_boost_system.m4 macro will fail due to
an unknown `__float128` type. Which will then in turn cause the spurious
and seemingly unrelated error above of not finding a sleep implementation in
boost.

A commit has been introduced into autoconf-archive with ID
d48ec2210047bbf17702700811c3f16a5369e06c that causes this issue. The fix
was meant to allow boost::system detection in case CXXFLAGS is containing
'-Werror' but conversely prevents proper compilation and thus detection
when the user uses the `-std=c++14` flag.

Compiling with clang-3.8 or afl-clang-fast is a way to get the fuzzer to
work in persistent mode.

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