OpenMesh 6.3 release

This will be the last release officially supporting C++98 compilers and building the integrated applications with Qt 4.

The update adds a workaround for an gcc optimizer bug causing segfaults when optimizing with '-O3'. If your gcc is affected (gcc 4.x and 5.x) OpenMesh will fall back to '-O2'. Our Python interface got some build system updates and we fixed several warnings.