libdecnumber: Fix warning due braced-groups within expressions

The libdecnumber uses the -pedantic compiler options that triggers
the compiler warning when braced-groups within expressions is used.
This patch removes its utilization within libdecnumber code.
libdfp: Disable C++ tests if compiler does not support it

This patch disable the C++ tests if the compiler does not have support
for std::decimal header/types.