bug#42702: catch2 build failure on ARM

[maxim . cournoyer]

On ARM systems, (armhf-linux, aarch64-linux), the catch2 package fails to 
build, due to a
compilation warning treated as an error:

[ 90%] Building CXX object 
CMakeFiles/SelfTest.dir/include/reporters/catch_reporter_xml.cpp.o
/gnu/store/ap7hgyv4rjqmhg4a6cb6cypsh3g1f5q4-gcc-7.5.0/bin/c++   
-I/tmp/guix-build-catch2-2.1.2.drv-0/source/include  -O2 -g -DNDEBUG   -Wall 
-Wextra -Wunreachable-code -Wpedantic -Werror -std=c++11 -o 
CMakeFiles/SelfTest.dir/include/reporters/catch_reporter_xml.cpp.o -c 
/tmp/guix-build-catch2-2.1.2.drv-0/source/include/reporters/catch_reporter_xml.cpp
/tmp/guix-build-catch2-2.1.2.drv-0/source/include/internal/catch_tostring.cpp: 
In static member function ‘static std::__cxx11::string 
Catch::StringMaker<char>::convert(char)’:
/tmp/guix-build-catch2-2.1.2.drv-0/source/include/internal/catch_tostring.cpp:202:21:
 error: comparison is always true due to limited range of data type 
[-Werror=type-limits]
     } else if ('\0' <= value && value < ' ') {
                ~~~~~^~~~~~~~