Eliminate 'constructor with 1 argument is not explicit' cppcheck warning

Eliminate 'constructor with 1 argument is not explicit' cppcheck warning

Note that -D CPPCHECK should be passed to cppcheck to activate
this workaround.

* include/gc_allocator.h (GC_ATTR_EXPLICIT): New macro (defined to
"explicit" keyword if at least C++11 or CPPCHECK, otherwise to empty).
* include/gc_allocator.h (gc_allocator::gc_allocator,
gc_allocator_ignore_off_page::gc_allocator_ignore_off_page,
traceable_allocator::traceable_allocator): Use GC_ATTR_EXPLICIT.
* tests/test_cpp.cc (A::A, B::B, C::C, D::D): Likewise.
* tests/test_cpp.cc (GC_ATTR_EXPLICIT): Define macro (as empty) unless
already defined in included gc_allocator.h.