Make the definition of WITH_ variable (end user actionable) a define
via a generic code
Move the WITH_DEBUG code into its own file Features/debug.mk
Replace USE_LTO into WITH_LTO and move it into Features/lto.mk
Move WITH_SSP into its own Features/ssp.mk
Add a variable to defined which one are activated by default (here SSP)
Reviewed by: portmgr (tcberner)
Differential Revision: https://reviews.freebsd.org/D35873
