stdenv: conservative undefined variables

pkgs/build-support/setup-hooks/autoreconf.sh

···
-preConfigurePhases+=" autoreconfPhase"
+preConfigurePhases="${preConfigurePhases:-} autoreconfPhase"
 
 autoreconfPhase() {
     runHook preAutoreconf

pkgs/build-support/setup-hooks/reproducible-builds.sh

···
 # This should ensure that it is deterministic across rebuilds of the same
 # derivation and not easily collide with other builds.
 # We also truncate the hash so that it cannot cause reference cycles.
-export NIX_CFLAGS_COMPILE+=" -frandom-seed=$(
+NIX_CFLAGS_COMPILE="${NIX_CFLAGS_COMPILE:-} -frandom-seed=$(
     outbase="${out##*/}"
     randomseed="${outbase:0:10}"
     echo $randomseed
 )"
+export NIX_CFLAGS_COMPILE