depends: add *FLAGS to gen_id

The depends cache should be busted when flags change, the same as any
other tooling change. Id also like to start passing *FLAGS into depends
inside the Guix env, which, without this change, doesn't bust the cache.

depends/gen_id

@@ -1,8 +1,9 @@
 #!/usr/bin/env bash
 
 # Usage: env [ CC=... ] [ C_STANDARD=...] [ CXX=... ] [CXX_STANDARD=...] \
+#            [ CPPFLAGS=... ] [CFLAGS=...] [CXXFLAGS=...] [LDFLAGS=...] \
 #            [ AR=... ] [ NM=... ] [ RANLIB=... ] [ STRIP=... ] [ DEBUG=... ] \
-#            [ LTO=... ] [ NO_HARDEN=... ] ./build-id [ID_SALT]...
+#            [ LTO=... ] [ NO_HARDEN=... ] ./gen_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
@@ -34,6 +35,13 @@
     echo "$@"
     echo "END ID SALT"
 
+    echo "BEGIN FLAGS"
+    echo "CPPFLAGS=${CPPFLAGS}"
+    echo "CFLAGS=${CFLAGS}"
+    echo "CXXFLAGS=${CXXFLAGS}"
+    echo "LDFLAGS=${LDFLAGS}"
+    echo "END FLAGS"
+
     # GCC only prints COLLECT_LTO_WRAPPER when invoked with just "-v", but we want
     # the information from "-v -E -" as well, so just include both.
     echo "BEGIN CC"