build: support LTO in depends

No Qt for now.

depends/gen_id

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 
 # Usage: env [ CC=... ] [ CXX=... ] [ AR=... ] [ RANLIB=... ] [ STRIP=... ] \
-#            [ DEBUG=... ] ./build-id [ID_SALT]...
+#            [ DEBUG=... ] [ LTO=... ] ./build-id [ID_SALT]...
 #
 # Prints to stdout a SHA256 hash representing the current toolset, used by
 # depends/Makefile as a build id for caching purposes (detecting when the
@@ -63,6 +63,10 @@
     env | grep '^STRIP_'
     echo "END STRIP"
 
+    echo "BEGIN LTO"
+    echo "LTO=${LTO}"
+    echo "END LTO"
+
     echo "END ALL"
 ) | if [ -n "$DEBUG" ] && command -v tee > /dev/null 2>&1; then
         # When debugging and `tee` is available, output the preimage to stderr