Don't make constructor of MappingQ explicit, after all. This sounded like a

Don't make constructor of MappingQ explicit, after all. This sounded like a
good idea but at least gcc 4.1 has a bug in which it complains that the
following code can't be compiled:
.....................
struct Mapping {
    Mapping ();
    explicit Mapping (const Mapping &);
};

void push_back (const Mapping &);

int main () {
  push_back (Mapping());
}
.....................
The error is erroneous, but we get hit by it some 20 times in the
testsuite. I'm also not entirely sure whether it's worth the effort of putting
a test for this in aclocal.m4 and making the constructor explicit only if the
test succeeds...

git-svn-id: https://svn.dealii.org/trunk@24167 0785d39b-7218-0410-832d-ea1e28bc413d