Do not override environment CFLAGS and CXXFLAGS
Do not override environment CFLAGS and CXXFLAGS

Looks like 5a05cf4063fc6ea666f3e24c60bd2e9e5526ef4e removed usage of
environment CFLAGS and CXXFLAGS by mistake. That change broke building
of nix on fedora core 23.

dtzWill/nix,peti/nix,bennofs/nix,peti/nix,ttuegel/nix,wmertens/nix,rycee/nix,peti/nix,dtzWill/nix,shlevy/nix,ttuegel/nix,rycee/nix,ttuegel/nix,dezgeg/nix,wmertens/nix,dtzWill/nix,NixOS/nix,pikajude/nix,layus/nix,ehmry/nix,shlevy/nix,ehmry/nix,ehmry/nix,NixOS/nix,peti/nix,ehmry/nix,peti/nix,layus/nix,ehmry/nix,bennofs/nix,shlevy/nix,NixOS/nix,ysangkok/nix,shlevy/nix,ysangkok/nix,dezgeg/nix,vcunat/nix,NixOS/nix,vcunat/nix,ysangkok/nix,ttuegel/nix,layus/nix,layus/nix,NixOS/nix,dezgeg/nix,dezgeg/nix,bennofs/nix,vcunat/nix,zimbatm/nix,rycee/nix,pikajude/nix,zimbatm/nix,pikajude/nix,bennofs/nix,wmertens/nix,rycee/nix,zimbatm/nix,dtzWill/nix,ehmry/nix,rycee/nix,layus/nix,wmertens/nix,zimbatm/nix,vcunat/nix,dtzWill/nix,shlevy/nix,dezgeg/nix