common: fix _FORTIFY_SOURCE redefinition error

On Gentoo _FORTIFY_SOURCE=2 is defined by default so passing
-D_FORTIFY_SOURCE=2 causes error. Undefining _FORTIFY_SOURCE
and then defining fixes the problem.
Merge pull request #23 from plebioda/fix-fortify-source-on-gentoo

common: fix _FORTIFY_SOURCE redefinition error