Merge pull request #1 from jlcox1970/master

Updates to make opensift compile on newer GLIBC distos
Fix libm missing link error by adding -lm
Merge pull request #7 from panovr/master

Fix libm missing link error by adding -lm
optimize computation of incremental sigmas. add clarifying comments.
Merge pull request #9 from chrislgarry/sig_optimization

optimize computation of incremental sigmas. add clarifying comments.
merge