Revert "Do not override environment CFLAGS and CXXFLAGS"
Revert "Do not override environment CFLAGS and CXXFLAGS"

This reverts commit 80ebd60e7ca3c48a19b94ddaeebb23182b703178. The
reason why we cleared CFLAGS/CXXFLAGS was because otherwise we get a
default value of -O2, which interferes with the defaults set in the
Makefile. (E.g. "make OPTIMIZE=0" should not pass -O2.)

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