bop-cpp test fails when using clang as the compiler

[Markos-Th09]

Unlike g++, clang uses the c++ mangling scheme for operators even when extern "C". For example

extern "C" {
UDT_C_DEFAULT operator+( UDT_C_DEFAULT const& lhs, UDT_C_DEFAULT const& rhs ){exec_bop( UDT_C_DEFAULT, + ) }
}

will become _Zpl when using g++, while clang will generate _ZplRK13UDT_C_DEFAULTS1_ (normal c++ mangling scheme)

I don't know how freebasic should handle this inconsistent behavior