g++ (macports) exception are not caught

This probably started happening when I upgraded my OS and whatnot.

I have code that throws a `std::string` (or something inheriting from it) and a `REQUIRE_NOTHROW` gives this:

```
libc++abi.dylib: terminating with uncaught exception of type std::__cxx11::basic_string, std::allocator >

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unittest_indexstruct is a Catch v2.3.0 host application.
Run with -? for options

-------------------------------------------------------------------------------
indexed indexstruct
basic construction
construction and simplify
-------------------------------------------------------------------------------
unittest_indexstruct.cxx:281
...............................................................................

unittest_indexstruct.cxx:281: FAILED:
due to a fatal error condition:
SIGABRT - Abort (abnormal termination) signal

===============================================================================
test cases: 2 | 1 passed | 1 failed
assertions: 161 | 160 passed | 1 failed
```

Here's my g++:

```
[imp:371] g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/opt/local/libexec/gcc/x86_64-apple-darwin17/7.3.0/lto-wrapper
Target: x86_64-apple-darwin17
Configured with: /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_gcc7/gcc7/work/gcc-7.3.0/configure --prefix=/opt/local --build=x86_64-apple-darwin17 --enable-languages=c,c++,objc,obj-c++,lto,fortran --libdir=/opt/local/lib/gcc7 --includedir=/opt/local/include/gcc7 --infodir=/opt/local/share/info --mandir=/opt/local/share/man --datarootdir=/opt/local/share/gcc-7 --with-local-prefix=/opt/local --with-system-zlib --disable-nls --program-suffix=-mp-7 --with-gxx-include-dir=/opt/local/include/gcc7/c++/ --with-gmp=/opt/local --with-mpfr=/opt/local --with-mpc=/opt/local --with-isl=/opt/local --enable-stage1-checking --disable-multilib --enable-lto --enable-libstdcxx-time --with-build-config=bootstrap-debug --with-as=/opt/local/bin/as --with-ld=/opt/local/bin/ld --with-ar=/opt/local/bin/ar --with-bugurl=https://trac.macports.org/newticket --disable-tls --with-pkgversion='MacPorts gcc7 7.3.0_1'
Thread model: posix
gcc version 7.3.0 (MacPorts gcc7 7.3.0_1) ```

The same code is perfectly handled by the Intel compiler and gcc on another platform.