guix: Map all guix store prefixes to /usr

Without ffile-prefix-map, the debug symbols will contain paths for the
guix store which will include the hashes of each package. However, the
hash for the same package will differ when on different architectures.
In order to be reproducible regardless of the architecture used to build
the package, map all guix store prefixes to something fixed, e.g. /usr.

contrib/guix/libexec/build.sh

@@ -223,6 +223,7 @@ CONFIGFLAGS="--enable-reduce-exports --disable-bench --disable-gui-tests --disab
 
 # CFLAGS
 HOST_CFLAGS="-O2 -g"
+HOST_CFLAGS+=$(find /gnu/store -maxdepth 1 -mindepth 1 -type d -exec echo -n " -ffile-prefix-map={}=/usr" \;)
 case "$HOST" in
     *linux*)  HOST_CFLAGS+=" -ffile-prefix-map=${PWD}=." ;;
     *mingw*)  HOST_CFLAGS+=" -fno-ident" ;;