Build using OpenMP

[ryanfb]

See this issue: alicevision/AliceVision#525

Will also need to depend against Homebrew's libomp.

[FunWithFlaggs]

@ryanfb could be that i solved it, not sure tho because i cant get a full build due to #5
Im now stuck at that problem while having OpenMP enabled (MeshDenoiser and MeshSDFilter do compile and link without error, which they did not before).

what i did:

1. git clone --recursive git://github.com/alicevision/AliceVision
2. mkdir build && cd build && cmake .. (which does succeed and detect OpenMP)
3. find build/ -name "flags.make" -exec sed -i -e 's/-Xclang -fopenmp/-Xpreprocessor -fopenmp -lomp/g' {} \;
4. find build/ -name "link.txt" -exec sed -i -e 's/-Xclang -fopenmp/-Xpreprocessor -fopenmp -lomp/g' {} \;
5. make

Then again stuck at #5