configure: better error message at compiler check
configure: check if -std=c++11 is supported
fixed_vector: de-activate ctor from size and allocator

For some reason, the relevant constructor from std::vector is not in
C++11.
However, it will be included in C++14.
Add explicit conversion to fix clang compatibility
Check for C++ defect core/1123

This results in a configure-error in GCC <= 4.7.2.
Clang doesn't have this problem.

see http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1123
and http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50043
and http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53613
and http://stackoverflow.com/q/11497252/500098

Closes #4.