Support case-sensitive file-systems + Add -std=c++11

[aman-tiwari]

In one place in the Makefile, include/HalfEdge.h is referred to as include/Halfedge.h. This breaks compilation on some filesystems with the error make: *** No rule to make target 'include/Halfedge.h', needed by 'obj/Edge.o'. Stop., as make thinks its a target as it cannot find the file. In addition, on my Ubuntu 18.04 machine, I had to add -std=c++11 to the CFLAGS.