IBM xlclang++ cannot build Catch v2.x

[datalogics-robb]

Describe the bug
IBM has nearly complete support for C++11 in 16.1.0 with the xlclang++ front-end, but is tripped up by what it considers to be an ambiguous call to std::abs

Reproduction steps
Build on AIX with xlclang++ using catch2 header <catch2/catch.hpp> (i.e. build catch2 tests with CMake)

Platform information:

• OS: AIX 7.1
• Compiler+version: XLC 16.1.0.1
• Catch version: v2.13.4

Additional context
With a patch to add a static_cast<> to the argument to std::abs Catch2 build

[ 50%] Building CXX object source_subfolder/CMakeFiles/Catch2WithMain.dir/src/catch_with_main.cpp.o
cd /home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/build_subfolder/source_subfolder && /opt/IBM/xlC/16.1.0/bin/xlclang++ -x c++  -I/home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/source_subfolder/single_include -qarch=pwr8 -qaltivec -qtls=global-dynamic -qpic=large   -O  -std=gnu++11 -o CMakeFiles/Catch2WithMain.dir/src/catch_with_main.cpp.o -c /home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/source_subfolder/src/catch_with_main.cpp
In file included from /home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/source_subfolder/src/catch_with_main.cpp:2:
/home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/source_subfolder/single_include/catch2/catch.hpp:11450:24: error: 
      call to 'abs' is ambiguous
        auto ulpDiff = std::abs(lc - rc);
                       ^~~~~~~~
/home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/source_subfolder/single_include/catch2/catch.hpp:11535:20: note: 
      in instantiation of function template specialization 'Catch::(anonymous namespace)::almostEqualUlps<double>' requested here
            return almostEqualUlps<double>(matchee, m_target, m_ulps);
                   ^
/opt/IBM/xlC/16.1.0/include2/aix/stdlib.h:851:18: note: candidate function
      inline int abs (int __n) { return __abs(__n);}
                 ^
/opt/IBM/xlC/16.1.0/include2/c++/math.h:733:1: note: candidate function
abs(float __lcpp_x) _NOEXCEPT {return ::fabsf(__lcpp_x);}
^
/opt/IBM/xlC/16.1.0/include2/c++/math.h:737:1: note: candidate function
abs(double __lcpp_x) _NOEXCEPT {return ::fabs(__lcpp_x);}
^
/opt/IBM/xlC/16.1.0/include2/c++/math.h:741:1: note: candidate function
abs(long double __lcpp_x) _NOEXCEPT {return ::fabsl(__lcpp_x);}
^
1 error generated.
Error while processing /home/robb/.conan/data/catch2/2.13.4/_/_/build/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/source_subfolder/src/catch_with_main.cpp.
gmake[2]: *** [source_subfolder/CMakeFiles/Catch2WithMain.dir/build.make:82: source_subfolder/CMakeFiles/Catch2WithMain.dir/src/catch_with_main.cpp.o] Error 1

[horenmar]

Merged the related PR.