Revert "GCC 4.6 compatibility: can't list-initialize array of classes."

It does too much harm on GCC 4.6 (all these constexpr constructors are
not constexpr now). We can disable that `-pedantic` warning for GCC 4.6
only and live with that.

This reverts commit 2d92d497d9a1b20c9ff83e386b58745955204d54.
GCC 4.6 compatibility: disable `-pedantic` warning.

The compiler doesn't like list-initialization of array of classes
(RectangularMatrix constructors). It is perfectly legal C++11 and both
GCC 4.7 and Clang accept it without notice.