style: move common optimization options to var

.build.sh

@@ -19,18 +19,20 @@ clone () {
 
 # optimizations
 set_cflags () {
+  opt="-Ofast -march=native"
   export \
-    CFLAGS="-std=gnu2x -Ofast -march=native" \
-    CPPFLAGS="-Ofast -march=native" \
+    CFLAGS="-std=gnu2x $opt" \
+    CPPFLAGS="$opt" \
     LDFLAGS="-Wl,-O3" \
     RUSTFLAGS="-C target-cpu=native -C panic=abort"
 }
 
 # in a separate fn because ring does not build with LTO
 use_lto () {
+  lto="-flto=auto -fwhole-program"
   export \
-    CFLAGS="$CFLAGS -flto=auto -fwhole-program" \
-    CPPFLAGS="$CPPFLAGS -flto=auto -fwhole-program"
+    CFLAGS="$CFLAGS $lto" \
+    CPPFLAGS="$CPPFLAGS $lto"
 }
 
 # make -j12