Changes to make slic3r build on Arch

Build failed with a compilation error (missing include file). Looks like a bug.

Second change, in Geometry.pm might not be a bug but some trick that didn't work for my system. I don't know perl well enough to tell. And I know, the perl version is not supported so that might be expected. It complained that XXX couldn't be found.

Perl 5.18.1
GCC 4.8.1
Arch Linux

After this I could build and run - but there are other errors performing some operations.