Respect system CFLAGS/CPPFLAGS/LDFLAGS
Respect system CFLAGS/CPPFLAGS/LDFLAGS

* only overwrite CFLAGS with intrusive "-g -O2"
  if they are empty
* always add non-intrusive warning flags
* use LIBS variable instead of LDFLAGS variable
* add CPPFLAGS and LDFLAGS where appropriate

jquesnelle/libntru,jl777/libntru,iblumenfeld/libntru,jl777/libntru,iblumenfeld/libntru,jquesnelle/libntru,jquesnelle/libntru,iblumenfeld/libntru,jl777/libntru