The problem is due to the Ruby version on macOS (Ruby 2.6).
How to enforce the package to use the brew installed version (Ruby 2.7)? Thanks

[ 45%] Building CXX object src/CMakeFiles/math.dir/CMakeFiles/math.dir/rubyRUBY_wrap.cxx.o
cd /tmp/ignition-math6-20200911-34806-1ontjo2/ignition-math-6.5.0/src && /usr/local/Homebrew/Library/Homebrew/shims/mac/super/clang++ -DMAC_OS_X_VERSION=1080 -Dmath_EXPORTS -I/tmp/ignition-math6-20200911-34806-1ontjo2/ignition-math-6.5.0/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/universal-darwin19 -stdlib=libc++ -DNDEBUG -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -fPIC -std=c++17 -o CMakeFiles/math.dir/CMakeFiles/math.dir/rubyRUBY_wrap.cxx.o -c /tmp/ignition-math6-20200911-34806-1ontjo2/ignition-math-6.5.0/src/CMakeFiles/math.dir/rubyRUBY_wrap.cxx
In file included from /tmp/ignition-math6-20200911-34806-1ontjo2/ignition-math-6.5.0/src/CMakeFiles/math.dir/rubyRUBY_wrap.cxx:889:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby.h:33:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/universal-darwin19/ruby/config.h:413:31: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
#define RUBY_ARCH "universal-"RUBY_PLATFORM_OS
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/universal-darwin19/ruby/config.h:414:35: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
#define RUBY_PLATFORM "universal."RUBY_PLATFORM_CPU"-"RUBY_PLATFORM_OS
^

/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/universal-darwin19/ruby/config.h:414:55: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
#define RUBY_PLATFORM "universal."RUBY_PLATFORM_CPU"-"RUBY_PLATFORM_OS
^

In file included from /tmp/ignition-math6-20200911-34806-1ontjo2/ignition-math-6.5.0/src/CMakeFiles/math.dir/rubyRUBY_wrap.cxx:889:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby.h:33:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:2111:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/intern.h:56:19: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
void rb_mem_clear(register VALUE*, register long);
^~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/intern.h:56:36: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
void rb_mem_clear(register VALUE*, register long);